The following slides were shown in the 11 December 2024 presentation for a Florida Wildflower Foundation Webinar Series. With limited time for each presentation, it’s not possible to explore much of the useful background or more thorough explanation of concepts. For that reason this webpage presents the webinar slideshow with annotations. Readers can also study Notes on Florida Wildflowers, a link leading to more complete discussion of wildflower biology.
The intro image shows as background an August view of a wet prairie, in Apalachicola National Forest, north of Sumatra. The dark fruiting heads are Rudbeckia graminifolia, the white are Eriocaulon. A yellow composite bending in the breeze is likely Helianthus heterophyllus. Scattered throughout are small fruiting bodies of Rhexia lutea and other plants. To the top left, you can see faint outlines of Lilium catesbaei.
This slide introduces images most people who studied intro botany in college will have seen. The figures are: a. Coleus growing tip long-section, b: Maize stem cross-section, c. Lilac leaf, main vein cross-section, and d. Lilac leaf blade, cross-section. All were taken from materials provided for instruction through Raven, et al (8th edition, 2013). There’s no attempt at this moment to explain these slides, rather the images are provided to demonstrate the cellular nature of plants, while also indicating to participants there will be some real Botanical materials involved in the discussion. Looking more closely into what students should know about the microscope preparations documented in these micrographs, however:
a. Using Coleus stem tip means we are dealing with a plant in the Mint family. The genus was, for a while, subsumed to the related Plectranthus, but was recently resurrected; so Coleus survives both as a common horticultural name (thus without italics) and a valid genus (italicized). Coleus is used for these long-sections because it’s easy to cultivate and readily available, producing plentiful soft active growing tips with square stems. Those characteristics are important; turning out microscope slides to sell for students (right, schools purchase teaching slides from a few companies, slides someone had to make by hand) means relying on plant material that predictably yields a usable product. Each Coleus glass slide a student sets on the stage of a microscope came from a stem tip that was “fixed” – which means it was killed quickly in an alcohol-formaldehyde solution that penetrates cells rapidly, controls physical deterioration, and makes it possible to infuse the tissue with wax. Once imbedded in a block of paraffin, the wax was whittled down to the correct shape and orientation to generate incredibly thin slices that would be mounted on glass slides, stained (typically with Safranin and Fast Green), then sealed under a glass cover slip. My guess is that the square stems on Coleus make it easy to align an embedded tissue sample such that turning out perfect micro-sections is straightforward. Moreover, Coleus seems always to take fixation and staining well such that the large nuclei in zones of active cell division (mitosis), which take the red Safranin stain, are readily seen by students. Safranin also stains cells further down the stem that show differentiation to xylem and fiber tissues. One subtle issue in teaching about stems using Coleus is that students probably don’t account for the fact that Mints have opposite leaves, as compared to many other plants (most other plants) with alternate leaves, and some even with whorled leaf placement. With all teaching models, students should realize other plants will be different.
b. The Maize (Zea) stem micrograph hides another secret. Glass slides for microscopes are small (26 x 76 mm, about 1 x 3 inches). Compared to the size of mature stems of field corn (i.e. maize), the stem section mounted to a microscope slide shows a curiously small diameter. That’s good, because a typical corn stalk would be too large to fit a stem xs (cross-section) on a glass slide, as well as too fibrous and thick to imbed with paraffin and cut into slices with a microtome. Technicians who make these slides must have a system to select very delicate or early growth that is suitable for these sections, but I’m not informed as to how this is achieved. The second issue relates to a standard way we differentiate grasses from their reedy cousins, voiced in the rhyme: “Sedges have edges and Rushes are round, but Grasses are hollow from tip to the ground.” We expect grasses, therefore, to have hollow internodes with solid nodes But Corn has solid internodes (as does Sugar Cane.) The image you see handsomely represents scattered vascular bundles we expect to encounter in Monocots, but students might be confused if they also learned grasses have hollow internodes – which most do.
c. The material used in this section, Syringa (Lilac), was selected based on generations of experienced material prep and slide making. You’d be hard-pressed to turn out such a model teaching slide using just any leaf. Here, amped up with Fast Green (which stains cellulose a light blue-green) and Safranin (which stains nuclear material and lignified cell walls, as in xylem and fibers, a rusty-red), students have the opportunity to orient themselves to the way vascular systems develop. As cells forming veins (vascular bundles) differentiate, xylem tissue (which transports water and nutrients from roots to shoots) develops closer to the center of a stem as compared to phloem (which stains pale blue-green,) which transports dissolved sugars (photosynthate). Phloem tissue forms parallel to xylem, but to the outside of a vein. That means phloem in veins that peel off to supply a leaf will be below the water-conducting xylem tissue, closer to the abaxial leaf surface. Understanding this becomes crucial in a student’s ability to construct a 3-dimensional mental image of internal plant anatomy. And real understanding requires studying many slides, sections through various zones of stems, leaves, roots, and other plant parts. Studying, and importantly, drafting their own drawings of samples proves to be the best way to make this leap in understanding the intricacies of plant growth, and makes it easier to understand formation of woody stems. Lacking comprehension of stem anatomy, students will face difficulties understanding how cambium makes the rings (secondary growth) we call wood.
d. The lamina (the flat blade-portion) of a Lilac leaf allows students to explore the structure and functional anatomy of a “typical” leaf. By typical, we mean a model leaf that shows the parts instructors need to talk about, the parts we can ask you to label on a test. Once again, it’s crucial to make teaching slides from material that reliably illustrates what students need to examine. That means a passable slide will showcase a discernible upper and lower epidermis, with one or more clear examples of stomata on the abaxial (lower) surface. Understanding “stomata” is fundamental to appreciating water management and regulation of atmospheric gases crucial to plant life.
Looking at a leaf right-side-up, of course means the specimen is mounted face down on the slide. One of the earliest challenges students have to conquer in learning microscopy is that images on mechanical microscopes are reversed – left is right and down in up. Moving a slide to the left causes the image to shift to the right. In order to becalm the confusion, leaf sections (as with the stem tip and main vein) are mounted upside-down so the student sees them right-side-up.
For Raven’s textbook, a perfect leaf x-section was selected, one in which the stoma (the mouth, or opening) of a stomate is clearly visible between the two guard cells that define it. And in this leaf, you can spot nice layers of “palisade” cells, the most chlorophyll rich cells, as distinguished from the “spongy” layer of chlorophyllous cells. That lower, loose layer allows gases (CO2 and O2, as well as water vapor) to circulate through leaf tissue and to pass in and out through stomata. Moreover, the jumbled-seeming placement impacts light scattering, increasing the chance a ray of light might be apprehended by pigments that harvest light energy for photosynthesis. You’ll also see cells that were alive when the tissue was fixed stain green, as contrasted with any fibers or xylem cells that are dead at maturity (cells functioning as non-living components, that die at maturity, are considered examples of programmed cell death, i.e. apoptosis). Students will also note that nuclei of meristematic cells are large relative to the cell size, though they will learn at a later point some cells do not require nuclei to remain alive. Phloem cells lose their nuclei, remaining enslaved to their nucleate Companion cells that retain the capacity to manufacture enzymes and other proteins
This first image in a many-slide sequence is taken from Robert Hooke’s 1665 publication Micrographia. One of the earliest, certainly the best-known publications of observations utilizing the emerging technology of microscopy, Hooke illustrates what he saw while peering through a sliver of cork. Cork is the heavily “suberized” bark of the Cork Oak, Quercus suber, a tree cultivated in its native Mediterranean habitats, typically in Portugal and Spain, and exported widely for waterproofing, insulating, packing, and bottling. Like many tree barks cork shows cell walls thickened with suberin (Graça, 2015), a waxy macromolecule produced by plants where water barriers are important. Cork Oak, however, is the master of suberin, producing heavy layers of spongy bark that are durable and long-lasting. Bark cells are apoptotic, dying at maturity; it’s programmed and necessary – the cells are purely for protection.
Hooke is the first person to publish illustrations of cork, and it seems the first to describe plant cells. Today, we understand that life is truly cellular, and that cells capable of growing and carrying on biological functions are living. It’s a touch ironic that the first cells illustrated, and the first application of the term “cells” to these units that are fundamental to all life on Earth, were based on bark, a dead tissue. It’s also somewhat fateful to look back and realize Hooke, as brilliant as he was could not possibly appreciate the basic importance of his simple discovery. At the very time his younger rival Isaac Newton knowingly was drafting an understanding of the basic physics that power our universe, Hooke quietly, almost naively unearthed the fundamental nature of all life on the planet.
Historically, Hooke and Newton worked during the critical cradle of modern science. The English crown had just been restored following disruption of the Cromwell era and a core group of brilliant scholars was chartered (by Charles II) as The Royal Society of London for Improving Natural Knowledge, the world’s earliest society dedicated to experimental science. Among other founding luminaries, you’ll find Robert Boyle (Boyle’s Law), John Evelyn (diarist, and author of Sylva), and Christopher Wren (architect of London following the great fire). Inspired through philosophies promoted by Francis Bacon and John Locke, theirs was, perhaps, the first generation of Europeans who acted on the challenge to embrace “empericism” – a drive to base knowledge on direct observation, experimentation, collection, and analysis of evidence (data that could be analyzed).
The next slide in our sequence (above) adds a figure (a Google on-line image) of a “model” plant cell, which is readily available, making it yet more problematic. The illustration copies science texts in exaggerating colors and relative sizes. Cell walls aren’t green, nuclei are clear, and cells aren’t colored unless there is pigment in the vacuole, or the plastids show obvious color. To make components more visible and easily labelled, however, graphic artists tamper with proportions of cell components, creating unrealistic assumptions for students. A central vacuole often fills much of the cell and nuclei of mature cells are much smaller as compared to figures. Moreover, cells with chloroplasts usually have more and smaller plastids, not just two or three that take up much of the space. Chloroplasts are moved around; in fact cells show a constant movement (flow) of components.
Most importantly, and wrongly, the drawing suggests that cells are simply pits in a solid cellulosic framework, like mesh wire. This is redolent of the drawing Hooke published in 1665, exacted using crude lenses without good control of light. We’ve learned a lot over the past 350 years, and it’s hard for me to believe someone at Google approved publishing this image. Cell walls are produced by individual cells, not as illustrated by Hooke in 1665 nor as proffered by Google in 2024. Hooke gets a pass. Google, with as much money and contemporary expertise as can be mustered, seems to have turned some AI loose to amalgamate a model image of a plant cell. Don’t believe everything you see published about plants, even when from a reputable source.
In succeeding slides I’ve borrowed a McGraw-Hill image (also from the internet), one that shows cell walls distinct from one cell to the other. But I removed the added color to remind us that cells are mostly clear; only pigments in plastids (like green chloroplasts) and water-soluble pigments we see in some vacuoles would bring color to a living cell.
The caption asserts: “Plants differ in that they: Grow primarily from localized points called meristems, which makes for linear architecture.” I showed this image early in the presentation, simply to bolster the contention that plants are made of cells. Here I hope to convince you that the Shoot Apical Meristem (SAM) is the fount from which new cells are generated, cells that will organize themselves as leaf primordia, lateral buds, and specialized tissues, reminding us the entire stem is a product of the organizing tip. Moreover, the SAM is a source of important Auxin (a hormone) that “regulates virtually every aspect of plant development.” (Taiz et al, 2018) When an animal browses plant tips, or the plant is damaged naturally, or shorn by a mower, or clipped or tipped, growth patterns change. Flowering may be aborted, or retarded.
In this slide I introduce Pinus longaeva, one of the Bristlecone Pines, that I was fortunate to visit in California’s Inyo National Forest. The image serves as eyewitness to the many thousand specimens that populate a few ridges in Western North America, plants considered the among the world’s oldest living organisms. The same California flora also includes Sequoia sempervirens, among the tallest plants on earth, and Sequoiadendron gigantea, among the most massive organisms known. The largest plant in the world, however, is not a tree, but a marine wildflower, a clonal colony of the Australian sea grass Poisidonia australis. One celebrated colony is reported as 8 km across with an estimated age of 100,000 years.
Photos added to the montage show plants I encounter in protected bayside marshes along the East end of Saint George Island. The plant keys out as a Gulf Coast Swallowort, but was difficult to tie down in that seemingly each book gave a different scientific name. The current accepted name, Pattalias palustre, might also be known as Cynanchum angustifolium or Sautera angustifolia, or even the outdated Ceratopegia palustris.
For the final image I drop in a photo taken by Shirley Denton and published in the ISB Atlas of Florida Plants. Descriptions of Allium canadense remind us the typical form generates bulbils at points that would more normally be flowers. The form that generates flowers only is given the name Allium canadense var. mobilense.
The next sequence of 4 slides introduces and distinguishes, very briefly, the four main groups of extant organisms considered “land plants” Work over the past century (Judd, et al, 2016) corrals plants green plants (which the book calls Viridophytes) as either Green Algae (Chlorophytes) or Land Plants (Embryophytes) Of land plants, the least complex are spore-bearing Plants that lack anatomically-approved leaves and vascular systems, the Bryophytes (Mosses & Liverworts), of which there are about 20,000 extant species.
The next stage introduces ferns and allies. These are spore-bearing plants with well-developed diploid phases, but still sporting independent haploid states. These plants develop vascular systems, and with ferns we see true, often large leaves (megaphylls). The group has a rich fossil history of extinct species, many that endured as giants over long epochs a half-billion years ago.
“Conifers and other Gymnosperms, c. 1100 species.” Believe it or not, the Atlas reminds us Florida’s native flora includes only 14 species of these seed-bearing plants, in the genera Pinus, Taxodium Chamaecyperus, Juniper, Taxus, Torreya, and the Cycad, Zamia. The photo was taken of a handsome Pond Pine (Pinus glabra) at Edward Ball Wakulla Springs State Park.
“Flowering Plants, c. 350,000 species” A photograph of Parnassia grandifolia introduces Flowering Plants, the Angiosperms, by far the largest group of extant plant species. All “Wildflowers” are Angiosperms. Flowering plants feature defined (but often very reduced) flowers, ovules borne in carpels, double fertilization (formation of endosperm), and a high occurrence of co-evolved animal pollination systems.
“So, what is a Wildflower – FWF: Any native, herbaceous flowering plant that existed in Florida before Florida existed as an entity – indeed, before Westerners introduced exotic plants.” This modified FWF definition of “wildflower” is both restrictive and a bit difficult. In the following discussion I expand that restriction on Wildflower to include suffrutescent plants, which are short-lived perennials with woody bases, and very light-weight, yet woody stems. Many genera that include herbaceous members also include shrubby representatives, sometimes described as herbaceous subshrubs. Hypericum is a clear example.
In the Florida Flora – Of 3317 plant taxa listed by the ISB Atlas as native: 408 (12%) are Bryophytes, 129 (3%) are Ferns & Allies, 14 ( 0.4%) are Gymnosperms, and 2777 (84%) are Angiosperms (Flowering Plants). Of those Angiosperms, 1887 (57%) are Dicots & 890 (27%) are Monocots. The great take-home in this slide is the small number of Ferns and Gymnosperms compared to Angiosperms, which constitute nearly 85% of native plant species. The slide also reminds us the ISB Atlas of Florida Plants is a tremendous resource. The opportunity to filter, sort, and download information from that database allows citizen scientists to analyze many aspects of the flora
The following two slides highlight some of our lovely native wildflowers, grouped by historical categories of Monocot & Dicot. Monocots pictured are Calopogon barbatus, Crinum americanum, a grass I am not competent to identify, and Pontederia. In the following slide, you see Asclepias connivens, Cakile, Clematis crispa, Berlandiera, and Buchnera americana.
In the third slide we updated classification so as to pull out plants considered more ancient than the true Dicots (Eudicots). This understanding developed as Systematists have discerned more about the origins of Angiosperms, realizing there’s a muddle of clades (lineages) out of which the Monocots and Eudicots arose as clearly coherent groups. I concocted the word “Protodicots” to group the diverse basic clades – there doesn’t seem to be a collective term otherwise. I added a “+” to Eudicots because some authers (Judd, et al, 2016) tuck the Caryophyllales (Caryophylls, Cacti, Spinach, Nyctaginaceae, Phytolacca, etc.) into a distinct clade just outside the major branch defined as the Eudicots (Roses, Legumes, Myrtles, Asters, Mints, Bignons, etc.).
Plants pictured include: Protodicots: Nymphaea, Nuphar, and Saururus; Monocots: Zigadenus, Uniola, and Cleistesiopsis; Dicots: Rhexia alifanus, Hibiscus moscheutos, and Hypericum myrtifolium.
The charts above tell several stories: 1. Native trees and large woody shrubs, though prominent in the landscape, are less than 20% of Angiosperm taxa. 2. The number of herbaceous protodicots native to Florida is modest, less than three dozen species. 3. The percentage of Monocots that would be considered Wildflowers is substantial because there are only very few “woody” monocots. Almost all native grasses and grass relatives are herbaceous, as well as all all Lilies, Orchids, and Amaryllids.
Taken from teaching materials available in Raven, et al (2013), images to the left depict Dicot germination and establishment, a Radish seedling and a figure of a sprouting Garden Bean (Phaseolus). The Radish photograph shows the emerging primary root, with a host of root hairs (which are typically seen in the young zone of root tips. A pair of seed leaves unfurl to mark emergence of the shoot. These two seed leaves (cotyledons) which were present in the mature seed now expand and green up in the sunlight.
The Bean diagram shows a plant that elongates the stem between roots and cotyledons (you’ll see that labelled as the hypocotyl), thus unfurling the cotyledons above ground. You see the first “true” leaves are simple and heart-shaped. The next set of leaves (not shown in the diagram) will be trifoliate, compound leaves with three leaflets associated with the Adult Phase of growth.
To the right are two illustrations of Maize (a Monocot), a cross-section of a seed and stages in germination. The Maize illustrations help explain the important and unique feature of monocot leaf and stem development, features that make a profound difference in the nature of these plants. Lacking cotyledons as we see in Dicots, Monocots like grasses and lilies send up a tubular sheath, a coleoptile, that emerges around the growing tip and is interpreted as as a second leaf, the first having been a curious, scale-like collar in the grain called the “scutellum”, interpreted as the single cotyledon (thus the term Monocotyledon). Succeeding leaves are produced in the same pattern, as a ring surrounding the growing tip. If the stem develops internodes, like a stem of bamboo, all is well. Each succeeding leaf sheathes the stem without issue. But consider the complication entailed if the growing tip generates new leaves with little to no stem growth. In that case you begin to understand the structure of a bulb (which will be diagrammed in an upcoming slide.)
To the far right is a stained long-section of a corn seed. Not to belabor this, it’s important to note that grains (a grain is also termed a “caryopsis”) have particular structure. In grains most important to society (wheat, rice, corn, oats, etc.), the starchy component occupying the bulk of the grain is endosperm, not embryo. Moreover, each seed of a grass matures united with its fruit wall, thus a grain is a 1-seeded fruit. The fruit wall, thin seed coat, and outer layer of endosperm (the aleurone layer, which generates enzymes that break down starch) together constitute darker cells we call bran. Polishing rice removes those layers, making it whiter, but less nutritious. Stripping the bran and germ (the lower right structure in the seed) from wheat grains yields endosperm that can be milled to white flour.
A micrograph of Coleus and a figure of Aucuba (Raven, et al, 2013) show development of leaves and internodes, illustrating the most common type of Dicot leaf attachment. When a leaf is shed, the abcission zone leaves a diagnostic leaf scar on the stem. Leaves of some Dicot families, such as Apiaceae, are spookily more similar to those of Monocots, showing broader, nearly sheathing bases. We also see encircling leaf stipules in Magnoliaceae and Moraceae. An overlay (in the second slide) reminds us the maize leaf, like other Monocots, sheathes and encircles the stem. Like Dicots, however, at the point where the leaf main vein connects with the stem, you’ll find an axillary bud.
In the preceeding slide I was hoping to expand the viewer’s concept as to what a leaf might be. The seed leaves and Shoot Apical Meristem develop together as the embryo matures; they are the first leaves that meristem generated. Bracts are non-photosynthetic leaves, ranging from the tiniest flaps of tissue to substantial bracts along a bamboo stem, or the spathe that subtends a Palm inflorescence, or the spadix of an Aroid. The diagram to the right reminds us that nodes are zones of leaf and bud production, zones that might generate roots. The node and internode constitute a building block, called a “phytomere” (or “metamere”),variably interpreted as a node and a full length of attached internode. In grass rhizomes, running along the soil, each node may become a fully developed plant segment called a “ramet”.
The same growing tip that makes a stalk can bolt from a rosette. We have many native rosette-forming herbs, such as the Composite, Chaptalia, and the annual Butterwort, Pinguicula pumila, a charming plant that germinates and forms rosettes in late summer into autumn we expect will flower the following spring and summer.
Plants develop many different growth habits, even several over the life of an individual plant. The slide above figures a bulb, and a cross-section of a red onion. Bulbs are geophytes that keep their growing tips underground until bolting for flowering. A bulb will develop photosynthetic green leaves, with enlarged underground bases. In onions, the season ends with production of succulent scale leaves that lack above-ground blades, serving as storage organs for flowering and the following season of growth. Development of a bulb, with its encircling scale leaves, is pretty much limited to Monocots
Having established that “leaves” come in many formats (scales, bracts, tendrils, even showing adaptations for carnivory) the standard approach to leaves is recognized in their dual roles as managers of water and atmosphere, which relates to leaves as a site for light capture and photosynthesis.
“The Wonder of Photosynthesis” begins with the true treasure at the end of the rainbow, reminding us sunlight comprises a visible spectrum of light, often taught as 7 hues, sometimes 6. Plants are green because they are onto this spectrum, collecting and using blue and red wavelengths more effectively than the greens. Our world is green because plants capture less green light, flushing the landscape with the leftovers. The reds and blues power what we call the Light Reactions of Photosynthesis.
In this sequence, we step quickly and carefully through a standard presentation of photosynthesis, updating the simple formula most of people will have seen, to a more robust formulation. Photosynthesis is incredibly complex. I certainly have a frail grasp of the intricacies, but have come to think of this as two distinct operations. One is the splitting of water and capture of light energy as chemical currency. The other is fixation of CO2 as carbohydrates. Textbooks reduce the process to a summary equation that elaborates a 6-carbon sugar, but it’s more realistic to think of cycles that attach one carbon at a time to a 5-carbon skeleton. The process then liberates a 3-carbon compound and reconstitutes the 5-carbon skeleton. Thousands of scientists have worked in hundreds of labs in countries around the world to characterize the basic processes. So much is known and understood, yet work remains to be done.
Basically, water steps in and out of the process in on-going ways. At one point we learn water is split, providing electrons that are energized by the capture of light in reaction centers. Those electrons are stripped of energy in complex cascades of reactions which invest the energy in an alphabet soup of compounds (NAD, NADP, ATP) It all sounds like agencies at Oak Ridge Laboratories.
Now cells, in their aqueous environment, have the currency to fix carbon (i.e. attach carbon to another carbon chain) available from carbon dioxide that diffused into the leaf through the stomata and was dissolved in a watery film surrounding cells. Through this process (called the Dark Reactions, because it isn’t confined to daylight hours), water is consumed and regenerated. It’s amazing researchers have been able to determine where the carbons, oxygens, and hydrogens start and end. But photosynthesis is one of the world’s most repeated organic reactions. We are told the base enzyme, Rubisco (Ribulose-1,5-bisphosphate carboxylase/oxygenase), is one of the most abundant proteins on earth. One source (yes, it’s an internet source) boasts their are 5 kilograms of Rubisco for each human on earth. But as human populations grow and plant habitats diminish, I’m guessing that will change.
The following slides make the argument that based on the ancient, unliving elements of Earth, Air, Fire, and Water, plants create the food, materials, habitats, and oxygen that make life possible on our planet.
HOPKNS CaFe – The story behind this slide will be expanded in the next presentation on Ten Big Ideas about Plants. The mnemonic on the slide is fairly self-explanatory, and a brilliant bit of botanical word-smithing. We have, ready at hand, a way to recall every element known to have some role in plant biochemistry. And the mnemonic also reminds us of relative importance (the macroelements are listed first), and even a hint as to origin (the first three are from air and water, the rest from media.)
“Roots and Shoots have different internal anatomies and roles: Anchorage, Water & Nutrient Intake, Mutualisms, and even Parasitism.” Micrographs and figures show the vascular system of roots develops as a unified core of xylem with fluted edges, nestling strands of phloem between. That central vascular system is barricaded in an impermeable cylinder known as the “pericycle” that facilitates the generation of what we call “root pressure”. Everything specialists have learned about root anatomy functions to promote the core root mission, which is to acquire and move water and nutrients to stems and leaves. Roots differ substantially from shoots, even on the surface;.there are no appendages or buds. Branch roots emerge in predictable locations along the vascular core, from inside the pericycle. It is this anatomy that defines a root, regardless whether it grows above or below ground. It’s not how you appear on the outside, but what you are inside that matters.
What this says to us is there’s a crossover zone, where the internal anatomy of the root system crosses over to the stem pattern. It also reminds us that there’s a bit of ledger-domain involved when roots emerge from stem tissue, such as is common at nodes along grass rhizomes. The secret here is captured by the word “totipotency”, which relates the capacity of plant tissue (we call unspecialized tissue parenchyma) to “de-differentiate” and take on a new future. A cell mass can form (a callus), and like the mass of cells that begins developing an embryo, the callus can form growing tips, root tips or shoot tips, or both, depending on circumstances and even hormonal activity. This is the reason propagators treat cuttings with IBA (Indole Butyric Acid), a functional analog to the important plant hormone Auxin (Indole Acetic Acid, IAA).
I selected a photograph of Lilium catesbaei for this slide to point out it’s a bit uncommon for a plant to generate a single flower (Trillium would also provide an example.) Most flowers are borne in inflorescences of more than one flower, some plants (Composites) offering heads of flowers numbering in the hundreds, others like Orontium and Typha with highly reduced flowers packed along stems to form a spadix or in Cattail, a dense spike zoned with female and male flowers.
Plants regulate development of the SAM (shoot apical meristem) to produce “initials” that will develop into differing kinds of appendages (leaves). For herbs, a juvenile state may quickly pass to the adult phase, which in many wildflowers may be a rosette of leaves. At some point, through some stimulus (or internal calendar), the growing tip changes course to generate a new branching structure with the singular purpose of creating flowers (and fruit). A flower is a short stem with a genetically-determined combination of specialized leaves (appendages) we recognize as sepals, petals, stamens, and carpels. These appendages differ from vegetative nodes in that axillary buds do not form. Moreover, the number and configuration of veins servicing each appendage is reduced in petals, stamens, and carpels. This is important to anatomists, and explains why the showy perianths of some flowers (Mirabilis) are judged to consist of sepals, lacking petals.
For as many kinds of Angiosperms that exist, there are just as many combinations and variations in flower parts. The following slides showcase samples of Protodicot, Monocot, and Eudicot flowers
You’ll discover very few Protodicots among native Wildflowers, less than three dozen. Commonly encountered examples are Nymphaea, Nuphar, and the Piperaceous Saururus. All three genera have worldwide distributions, but with limited numbers of species. Nymphaea (Waterlily) shows the greatest variation, with about 70 species.
The related Nuphar (Spatterdock), long thought to constitute a single variable species, embraces as many as 20 species according to contemporary taxonomists. Flora of Florida treats the native plants as three subspecies of Nuphar advena. Investigating the flower of this protodicot, we learn that like so many early flowers, the sepals grade into petal-like tepals. In this plant particularly, the outer and inner (yellower) tepals are considered sepals, while the petals are much more similar to anthers. The diagram below, from Flora of Labrador, helps explain that structure.
Saururus is treated as S. cernuus, the single species native to the Western Hemisphere, as compared to the only other recognized species, Asia’s Saururus chinensis.
Many of our wildflowers are Monocots. Examples pictured are the Amaryllid Crinum americanum, the Melianthaceous Zigadenus glaberrimus, and the orchid Calopogon barbatus. What’s missing from this spread (a shortage I want to address) is the largest group, the Poales. This “clade” includes the Grasses (Poaceae) of course, but also the Bromeliaceae, Cyperaceae (Sedges), Eriocaulaceae, Juncaceae (Reeds), Mayacaceae, Typhaceae (Cattails), and Xyridaceae. It almost seems botanists took the plants that are hardest to key out and shoehorned them into a single order. It would be a life’s challenge to gain mastery of this order. Just examine the small inventory of books on Florida’s wildflowers and it become evident botanists in general avoid the complications of grasses and sedges.
The Eudicots+ probably represent plants most people imagine to be wildflowers – The Mints (Lamiales), represented by Macbridea, are allied with other Asterids, including Asters, Umbels, Gentians, Nightshades, and many others. A major clade, the Rosids, which includes a quarter of flowering plants (groups such as the Roses, Legumes, Oaks, Myrtles, Crucifers, Mallows, etc) are represented here by Hypericum, Crocanthemum, and Rhexia. The + tells us the Caryophylls are distinct enough to be segregated from Eudicots by some botanists, while others have good reason this branch should be included. Represented here by Drosera, the very well-defined clade encompasses Cacti, Phytolaccas, Plumbagos, Portulacas, Four O’clocks, and of course the Pinks (Caryophyllaceae).
The following two slides give closer views of floral parts, using Hibiscus moscheutos (the Mallows are a branch in the Rosid clade) and Ipomoea sagittata (the Bindweeds are grouped with Nightshades, which are Asterids). These show differing configurations of stamens, as well as relative sizes of stigma to style.
This last slide of the sequence focused on Composites as examples of “pseudoflowers” – tightly clustered groups of flowers with the appearance of a single blossom. These plants help us understand the complexity of attempts by plant morphologists to create a perfect framework that classifies inflorescence branching types. Liatris, for example, appears to form a spike, except we are looking at heads of flowers, thus botanists resort to “spiciform” (spike-like) as an invented but useful term. As soon as a rule is established, the next plant encountered proves an exception.
This slide and the next shift the conversation to pollination and breeding systems. The above collage above introduces vectors (wind and animals that move pollen) and the concept of syndromes. Botanists associate suites of flower and inflorescence characters with particular kinds of vectors. The photos illustrate a few syndromes:
Hummingbird syndrome is associated with red, tubular flowers, rich in nectar and exposed so as to provide good access for a hovering bird.
Crepuscular Moth syndrome includes white to light-colored flowers with long narrow tubes that limit visitors to those with a considerable proboscis. These flowers open freshly for evenings, produce good quality nectar, and are often fragrant during cooler hours.
Bee Syndromes vary, with this cluster of images focusing on flowers that require some form of manipulation (the Partridge Pea), a particular skill associated with bees. Bee flowers are often zygomorphic, even colored in the blue hues (the Lupinus). They can be broad, with a gullet that requires an effective pollinator to enter the flower. An obligately-bee pollinated flower might be visited by butterflies, but with no impact on pollen movement.
Wind-pollinated flowers are common among the Grasses, which show highly reduced flowers, versatile stamens, copious pollen loads, and feathery stigmas.
The reason flowers exist, clearly, is to achieve some level of outcrossing, which benefits populations by maintaining genetic diversity and giving rise to novel combinations. Most wildflowers, however, are perfect (an individual flower or a single plant will have both male and female components), raising questions as to how out-crossing is achieved. Apart from pollinator selection that opens greater opportunity for outcrossing, there are safeguards, i.e. “strategies” that increase the likelihood of outcrossing, even prohibiting self-pollination. Lythrum, for example, is distylous, which means each plant will make one of two different floral configurations. One plant will make flowers with short stamens and long styles, while another will make the reverse. This positions pollen on visitors such that pollen from one form is more likely to contact the stigma of the alternative floral configuration. Plants might also change timing, for example maturing pollen before the stigma is receptive (protandry), or the reverse. Timing is frequently related to herkogamy, specific structuring and positioning of stamens and stigmas that makes pollination more efficient while limiting the possibilities of self-pollination. Some plants, such as most Pinguicula pumila and most Composites are obligate Outcrossers, which means pollen produced by a plant will not successfully mature and fertilize carpels of the same plant. I show Sagittaria, which is monoecious, producing female flowers early on a stalk, with male flowers maturing later in time (basically protogynous). You’ll know that may Cucurbits are also monoecious, producing male flowers on younger stems with female flowers showing up on more mature stems.
Pollination is only the first stage in reproduction. Once a pollen grain arrives on a receptive stigma, there’s quite a series of hurdles. The grain is bereft of resources, basically only carrying two sperm nucleii. To deliver the sperm cells to the egg sac, a grain must receive moisture and nutritive support from the carpel. It germinates on the stigma, then grows (tip growth) down through the style to reach the ovule. That distance can be very short, but in flowers like Lilium catesbaei and Crinum americanum, its more than an inch. In commercial corn crops successful pollen tubes growth exceeds 40 cm. That’s actually astounding, especially when you see the frailness of a pollen tube, about a tenth the diameter of a human hair. But it does work. Every seed of an Angiosperm that results from normal fertilization testifies to success.
Figures in the following slides (from Taiz, 2018) show a pollen tube with two nuclei, one that unites with the egg to produce a zygote (the first cell of a new plant), and a second that unites with other nuclei to generate a seed storage tissue called endosperm. Some plants, such as grains, develop a considerable mass of endosperm, whereas in others (Legumes for example), endosperm development is insignificant.
The next few slides complete the life cycle with seed and fruit development. Examples include Platanthera chapmanii and Zigadenus glaberrimus – an orchid that shows inferior fruit and the Zigadenus, with its 3-locule superior ovary. In the following slide I show Callicarpa (arguably not a wildflower), a shrubby mint that develops leathery berries, rather than dry capsules characteristic of most members of the mint family. I also include Sarracenia flava, another Asterid (but in the Ericales branch), that develops a dry capsule.
The following slide shows Hibiscus (a Mallow in the Rose Clade), with the typical capsule. One wonders how these plants, with their hard dry seed, are dispersed. The H. moscheutos appears to invite birds to harvest seed. But at the season’s end this year I collected few seed of the related Hibiscus coccineus and set them in a vial of water, where they floated for 4 months. Just after Thanksgiving, the seed germinated while in the water, helping me understand how these plants, common in freshwater marshes, can distribute seed over longer distances.
The final slides show Seymeria, Pattalias, and Lupinus, all of which produce their particular dry capsules from which seed are dispersed, leading to a concluding slide reminding us that Composites are different. They are among many plants that generate 1-seeded fruit that do not open to disperse seed. Rather the dry fruit is the dispersal unit, often with a parachute of pappus filaments, or even barbed projections that make these plants hitchhikers. The same is true of Grasses, both with structure and dispersal. Thus the two largest groups of wildflowers disperse seed that are, in reality, whole fruiting structures. Not so important in many regards, but significant in understanding flower and fruit structure, and in realizing the fruit are important ways to recognize and define plant groups. “By their fruit, ye shall know them.”
The closing slide acknowledges assistance I receive from so many sources. Marina, Emily, and Rose, of Florida Wildflower Foundation, organized this lecture series and hosted the Zoom sessions. Staff of many public properties, Apalachicola National Forest, Box R WMA, Tate’s Hell State Forest, and Torreya State Park provided permits, access, and guidance in my studies. Alan Franck, of Florida State Museum in Gainesville, provides on-going feedback, as do Ethan Hughes, Florida Native Areas Inventory, and David Roddenberry, Florida Native Plant Society. Danielle Rudeen, long my assistant at Huntington Botanical Gardens, continues to help through locating valuable information and resources.
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