APPROACH: KNOWING BEANS
When I talk with Gardeners about pure botany, the conversation skews toward issues related to productive gardening. Discussion concentrates on structure, with the goal of empowering gardeners to make sense out of any plant through understanding the root-stem-leaf basis for all plants. Students begin with a thorough examination of bean embryos, and the nature of legumes as a type of fruit. We explore the origin of the legume as a pistil in the bean flower, and determine which tissues are root, stem, and leaf. We then compare this to a selection of the kinds of vegetables and fruits one can grow and purchase forfood.
To work yourself through this discussion, it will be useful for each participant to have the following items:
- a few red kidney beans or Pintos (the seed) that have been soaked in water for at least 24 hours.
- a fresh snap pea (the entire fruit)
- a fresh green bean (the entire fruit)
- a raw peanut (in the shell)
For the presentation, a quick trip to the grocers can provide much of the material needed. In season, the garden yields even more exciting examples. (See a list of suggested materials at the end of this section)
The Lecture Transcribed: My thoughts on “Botany for Gardeners” continue to evolve, and I have begun the discussion in many different ways over the years. For this exercise, let’s begin with a Snap Pea. Crack it open carefully, along the most curved edge. The pea (which is a kind of fruit called a legume) will splay out to a leaf shape with seed attached along the main vein that runs down the center. You are, indeed, looking at a leaf – one that can be described by many terms.
The entire pod is the mature pistil from a pea flower, which makes it, by definition, a fruit. In flower, the pistil (this single, specialized leaf) includes an ovary, its style, and stigma (or stigmatic surface, where pollen lands and germinates.) After fertilization, the ovary develops into the pod, but you still see remnants of the style and stigma. Now opened, laying before you are the peas in this pod – attached where they were formed as ovules on the line of tissuecalled the placenta.
Compare the Green Bean to the Pea, open it just as you did the pea pod. If you are successful in determining which side to open, the bean fruit will also part like a book, and lay down as did the pea pod. Though differently shaped, it is really easy to see how comparable these two fruit are. Each is equivalent to a single leaf – the leaf that formed the pistil (ovary, style, and stigma).
I find it really useful to think about this for a minute, and worthwhile to make a cross-section of a second pea pod as well as another bean. In cross-section, you will see that each pod is made of a leaf that wraps around a chamber in which the ovules are formed. This single leaf is a carpel. The fact that the ovules (and seed) form inside a closed chamber is one of the defining characteristics of flowering plants across the board – and the reason we call the flowering plants angiosperms (angio = chamber; sperm = seed.)
Now go through the same exercise with a raw Peanut. (You can often find raw peanuts in the shell at grocers. Roasted peanuts will do if raw ones are not available.) The peanut is a fruit also. It isn’t green, and it formed underground, but those characteristics are un-important, superficial. This is the mature pistil from a flower, inside which ovules formed and developed into seed after fertilization. The Peanut is in the bean family, so just as with the pean and bean, we call this pod a legume.
There are now three different legumes before you – different yet obviously morphologically similar. Each is a fruit (a mature pistil) made of a single carpel. And each bears one to several seed along a band of fertile tissue called the placenta.
Now it is time to dissect one of the Kidney Bean seed (Pinto). If you still have some dry seed available, compare them to those that were soaked. The soaked Kidney Bean seed are larger, heavier, and softer from having imbibed water [See examples in the discussion of Beans,Chapter 9.]. They are mobilizing resources and preparing to germinate. Each seed is covered with a red skin, which is the testa, also called the seed coat. The testa is part of the ovule and has the same genetic makeup as the motherplant.
Gently peel off the testa, and allow the bean seed to open on the surface in front of you. The cream-colored bean seed is the embryo, a genetically new plant. It probably fell onto the work surface into two parts, which are its two seed leaves (also called cotyledons). If you are fortunate, the young root and growing tip remained attached to one of the leaves, and are visible with normal eyesight. It is very useful to have a magnifying glass, a handlens, or a dissection microscope to study this structure – but not necessary.
The embryo is visible to the naked eye. It is an intact and totally competent plant. You can see its root, stem, and leaves, which are all of the basic kinds of organs the bean plant will ever make. Indeed root, stem and leaf are the basic organs that make up any plant; everything else (flowers, fruit, thorns, spine, etc.) is an elaboration of one of these three organs.
Now open a raw peanut seed. Things look different, but are exactly parallel. The two halves of the peanut are its seed leaves, and attached to one of them you will see the remainder of the embryo – a growing tip and root. If you plant some of the Kidney Beans in a pot along with the Peanuts, it is instructive to watch germination, which is only a few days away. The differences in embryo structure play out as particularities in germination style, with the Kidney Bean pulling its shoot tip and seed leaves from the ground and the peanut pushing the seed leaves and growing tip straight out…..
Given time, patience and a dissection microscope, you can investigate the seed in the snap pea pod. Remove one of the peas to discover it is also an embryo of similar structure. Of course, you could have acquired a few dry pea seed and soaked them in water (along with a few huge Lima Beans and other beans) to study a bit more easily.
Through knowing these beans and peas, you also know a lot about plants generally. Almost all flowering plants that have two cotyledons structure their seed and germinate much like the beans. This is powerful information for a gardener, in that along with site selection, soil prep, cultivation, and many other tasks, planting seed is a core activity.
In the end, however, produce gardening is about harvest. Botanically, what is the product? How do you reach the needed yield? When do you harvest? And how must you handle and process the fruit of your labors?
So what are we growing? If everything a plant makes is root, stem, or leaf, then obviously we are growing roots, stems, and leaves. A good exercise for gardeners is to compare a host of fruit and vegetables to determine what is what. Baked into this has to be more information on flowers and fruit, because the make-up of fruit does not so readily reveal itself without examination and comparison.
Once I believe a class has had enough of beans, it is time to break out as many kinds of produce as I am able to provide – starting with plants we grow for their leaves, because most people are fairly confident in knowing a leaf when they see one. A simple spinach leaf, or a massive collard leaf provides an unambiguous example. Hold one up to the light and think for a moment about the color and character of the leaf. It is variable in density, most especially because there are so many veins coursing through – larger veins in a pinnate pattern (with the same pattern as a feather) and the smaller branching veins having a reticulate pattern (intersecting and organically grid-like). The veins have to branch sufficiently such that every cell is within a functional distance of water and nutrients, and for exporting sugars. That of course relates to the main activity of this rather normal leaf as a photosynthetic organ, which is the reason the leaf isgreen.
Contemplate that for a minute. Why green? Why are plants green? The simple answer is that plants are green because chlorophyll is green, and there is enough chlorophyll in these leaves to make them that color. But the somewhat counter- intuitive answer is that chlorophyll is green because it absorbs red and blue light – reflecting other colors, which includes all of the green light. With green at the center of human color perception, we perceive green more acutely than other colors. With red and blue filtered out (absorbed) by chlorophyll, the leaves come across to us as overwhelmingly verdant. Leaves reflect/transmit other colors, but as long as chlorophyll is abundant, those spectra are swamped by the preponderance of green light.
But on to more about leaf structure….. The spinach and collard leaves each have a large flat surface, the blade, and a stalk, the petiole. Lay the leaf or leaves on a table and begin to plunder other vegetables for comparable forms. I like to pull out a head of red cabbage, a head of lettuce, and a stalk of celery.
Now take an intact leaf from the red cabbage and lay that next to the others. Cabbage leaves are also almost all blade, with very little petiole. Cut the head of cabbage in half, from the tip down through the stem (this is a long section). The white cone-shaped core, the part cooks toss in the compost, is all that exists of the stem. (at least right now). Everything else is leaf; it is a lot of closely spaced leaves spiraling around a very short stem, making a plant form gardeners call a rosette.
Many vegetables we grow that are in the cabbage family (the Brassicaceae) make rosettes in their early growth stages – cabbages. kale, turnips, mustard, collards, broccoli, cauliflower, radishes, etc Any of those could be brought to the cutting table and compared for further edification.
But this growth pattern characterizes other plants we grow for leafy produce. Lettuce, which is in the daisy family (the Asteraceae) and not closely related to cabbage at all, also forms rosettes in its juvenile stage. Peel off a leaf and compare it to the cabbage, different in texture (and taste), but similar in shape, with hardly any petiole. A more useful comparison is to cut the head of lettuce in long-section just as you did the cabbage. For two plants that are so unrelated, they certainly have similar structures, each with a short stem supporting a rosette of leaves.
When I do this for a class, it’s nice to splurge and purchase one of those high-end lettuces that comes packed in its own plastic house. Those are marketed as “living” – which is absolutely true. They are alive. But any lettuce that is still edible is living, as are the head of cabbage and the all fresh leaves you would bring to the table. Other lettuces (like Romaine, or Butter Lettuce) can be dissected for comparison and thoroughness.
Now take out a “stalk” of celery (Chapter 5 Section 3 treats green grocer terminology under Political Detente) – a relative of the carrot (in the Apiaceae). Pull off a single “rib” and compare that to the other leaves. Unless you managed to lay hands on an un-circumcised celery stalk, you will already be missing some of the leaf blade, but it should be obvious through comparison that the rib of celery is one leaf, with a petiole that is greatly developed and a leaf blade that is dissected into several segments (compound). Now cut the remaining stalk in half (long-section) as with the cabbage and lettuce. Celery is a rosette also, but the edible portion is mostly petiole.
There are many other examples that can be explored, which makes a worthwhile pursuit. But the point about leafy vegetables should be clearly established. As the preferred source of leafy greens, gardeners have selected plants that produce tasty, non-toxic leaves in nice, quickly-growing clusters.
We will set aside stems for a minute because this is a good moment to speak of roots. If every example examined so far came straight from the grocers, the only roots seen will have been those included with the “living” lettuce.
In the sample list, however, we do have bona fide roots, because carrots (and sometimes even turnips and beets) are easily available. With the previous samples of leaves and plant long-sections laid out before you, follow the same examination and dissection with a carrot (note this should be a carrot that retains its leaves). Compare the carrot leaf to others, most particularly to the celery. It is a miniature celery leaf, and if you ever grew celery without providing a lot of water, you might find those leaves nearly indistinguishable from carrot foliage. Once you have cut the carrot root and short stem in long-section, it becomes clear the carrot has the same rosette organization for its stem and leaves as the celery, lettuce, and cabbage. But this is mostly lost information because we grow carrot for the fleshy primary root.
If you have whole Beet plants available, compare those also. Like the carrot, beets are pretty much all taproot. Now examine a Radish, which is in the mustard family along with cabbage. Repeating the same procedure as before, you see the rosette strategy yet again (see Chapter 5, Section 3 – An Annual by Any Other Name), though in the case of red radishes, the edible portion is a bit complex anatomically, since it forms where the stem changes to root tissue. With this exercise we arrive at consolidation for an overarching concept: Many of the plants we grow for leafy vegetables and simple root crops are short-lived, once-flowering plants that we harvest before they begin the flowering process. In each of these instances, the plants become inedible when they begin to “bolt” – which is the term gardeners use when the short stem of rosette plants quickly elongates and develops into a floweringstalk.
Having covered leaves and roots, it is time to investigate stems more completely. The most obvious vegetable crop we grow for its stems would be Asparagus. And many people harvest stem tips of various garden crops, such pea shoots or the growing tips of pumpkins. But most stems we consume as vegetables will be confused with roots.
Chief among stems masquerading as root crops is the Potato. I seem to cover this issue almost every time potato comes up as a topic, but in this instance it is worth another emphasis. In class, I provide a potato for each student to examine. Russets will do fine, but the specialty fingerling potatoes are even more easily comprehended as stems. I ask every student to use a felt marker and color each potato bud (eye), beginning at the growing tip. The buds are evidence a potato is a stem, because roots do not make buds. Compare the organization (pattern) of the buds to your understanding of stems (to the asparagus, if that is handy). A potato is just a big. fleshy, underground stem – the very definition of a tuber.
Other confusing vegetables include the true Yam, Dioscorea. Yam is a curiosity as to its development, but anatomist determined many decades ago this is not a root, rather yam is stem tissue, thus we can call yam a tuber.
If you have grown Sweet Potatoes, take a note. A Sweet Potato is unrelated to both the yam (Dioscorea) and the regular potato (Solanum tuberosum). Sweet Potato is the starchy sweet and tuberous root of a vine (Ipomea batata) in the morning glory family(theConvolvulaceae). When I was a kid in South Alabama, Sweet Potatoes were common enough as to make the word “potato” ambiguous, so we always distinguished the other potatoes, Solanum tuberosum, as Irish Potatoes.
MONOCOTS: With examples so far, we have conquered seed, germination, and plant parts (roots, stems, and leaves) for most flowering plants. By that, I mean for dicots. Most textbooks get worn out on dicots and fail to explain that Monocotyledons are a different story. It is the alternative life style and growth habit of monocots that makes things like bulbs and onion-rings possible. The difference begins in the seed. Most monocots have seed that do not conform to textbook examples, which are usually beans or other dicots. And they do not germinate or grow like dicots either.
A useful, somewhat familiar and available example would be grass seed. A few seed of almost any grain can be germinated in advance, or sometimes sprouted grass can be purchased in small containers (germinated wheat grass is commonly available for people to use for juicing). Wheat (and other grass) seed are different from the get-go. The Wheat “berry” you purchase for grinding, sprouting, or cooking is, anatomically, a single-seeded fruit. This is not easily detectable, because in grains the difference between fruit wall and seed coat is imperceptible. Most people are happier just thinking of a grain as a seed. And for me it is a technicality- battle not really worth fighting.
The bulk of each grain is a nugget of endosperm – an intriguing and starch-rich tissue that does not develop in most dicots but is a big deal for monocots. It is the wheat endosperm that we mill to produce white flour. Whole wheat flour contains the endosperm and everything else that makes up the one-seeded wheat fruit – that means it has the bran (outer seed coat and fruit wall) and the germ (the embryo).
When you examine wheat grass sprouts, you will see that they look nothing like a germinating bean. The embryo breaks out of one side of the grain, sending a tubular shoot upward and several small roots spreading downward. That tubular shoot is interpreted as a single seed leaf, but botanists give it a separate moniker; we call it a coleoptile.
Unlike the leaf of a dicot, the coleoptile forms as a cylinder, completely surrounding the growing tip. That circumcisal pattern of leaf production continues throughout growth.
Every new leaf forms as a ring around the tip, which makes grasses and the other monocots structurally very different from dicots. This tubular-leaf formation may seem like a minor deal, especially when you consider that most textbooks do not give the monocot growth pattern much emphasis. But if you can really turn attention to this truly alternative system of assembling a plant, it will greatly improve your ability to comprehend why Onions and Sugarcane, Lilies, Gingers, Orchids, and Palms defy so many rules. They keep to the core plan, only making roots, stems, and leaves. And just as with dicots, at each node where a new leaf is born there is also at least one bud produced. But if your entire understanding of germination and plant growth is based on dicots, these plants will never really make sense.
Follow the growth pattern for a few more iterations. A second tubular leaf is formed, which in wheat is the first green leaf. It emerges from inside the coleoptile; we say that the coleoptile “sheathes” the succeeding leaf. The third (and each successive) leaf follows that same pattern, emerging from within the previous tubular leaf, which is sheathing. This is a bit crazy – like how are things going to work out? How can leaves continue to emerge inside one another.
In wheat, resolution is quick in coming. The growing tip begins to make a stem that pushes upward through the sheathing leaves. New leaves will be produced, but they will encircle a stem that has risen above the earlier leaves. Check out bamboo or sugar cane to see this growth form.
Other monocots delay emergence of the stem. Banana plants, for example, create a “tree” that is all sheathing leaf bases, culminating with a flowering stem that pushes up through the center. That is impressive, but bulbs are the more notorious. The list of study samples includes both Green Onions and Bulb Onions (red, white, or brown). Make a quick examination of the Green Onion as compared to the Wheat seedling. If you have plenty of samples, cut one in cross- section and the other in long-section. The long-section shows a basic organization with which you are now familiar – there is a short, stubby stem that continues to produce leaves without itself elongating. It resembles cabbage, lettuce, and celery, something of a rosette strategy. The cross-section reminds us how different onions are; each leaf is a cylinder, with the outer leaves being the first to have been formed (therefore, the oldest). This means that once they have been formed, these sheathing leaf bases must retain growth capacity, because they continue to expand in circumference as new leaves are created to the inside.
Now, using a couple of Red Onions (or Yellow, Brown, or White), make the same long-section cut in one specimen and a cross-section of the other. Pretty comparable to the Green Onion…, yes? Except…, where are the green leaves?
People who have grown onions can answer this question. When you are looking at a scaly Red, White, or Brown Onion, the green leaves long since withered and turned brown. The only evidence of the previous existence of green leaves that grew above ground is papery leaf husks that remain surrounding the fleshy bulb leaves. What you are holding is a bulb, made entirely of the short stem (with its remaining roots) and its scale leaves. So this plant makes two kinds of leaves. It starts growth with large green leaves that form above ground. Once those green leaves are operating at their photosynthetic peak, the next leaves that form are these scale leaves – fleshy sheathing leaves that remain underground and never form green blades. These scale leaves constitute sugar and starch storage sites for future production of the stem that will bear flowers and fruit. And a gardener will know that when this flowering episode happens, the bulb is no longer edible. Once again, as gardeners we are harvesting the young, intervening in a life cycle to capture stored food before the plant utilizes it.
Normal photosynthetic leaves of the bulb onions are not available for inspection. But you can examine the Green Onion leaves and note those leaves do something bizarre. Each emerges as a sheathing tube that breaks open where the succeeding leaf will emerge. The leaf then returns to its tubular shape. That happens time after time, and is different from the wheat seedling, in which once the sheath breaks open to form a flat blade, that laminar form is kept. It also differs from some other onions, such as Leeks. Examine a Leek sometime – each leaf emerges as a cylinder, but breaks open to a flattened blade that stays laminar.
Given these simple models for growth in monocots, we have a base from which to understand related plants – not simply other grasses and lilies, but also the aroids, gingers, orchids, palms, and sedges. Most people walking the streets around you do not have that appreciation. Granted, most people see little reason to suspect there is magic to consume in plant structure. They would likely be surprised that to other folk, knowing how plants grow and develop is satisfying and useful.
Flowers and Fruit: There are some flowers gardeners harvest for culinary use, but few constitute a serious addition to human diets. When I think of flowers and food, my first thoughts relate to garnishes such as Pansies, Nasturtiums, and Alyssum – some tasty, others simply innocuous. And there are Pumpkin or Squash flowers, or even Daylilies, occasionally stuffed, sometimes eaten as buds. Commercially, the buds of Caper plants are sold pickled, and some flower buds (such as Daylily buds in Chinese food markets) are available dried. And if I don’t mention Broccoli and Cauliflower, people will chide me.
But the principle interest most produce gardeners have in flowers is the Jekyll and Hyde relationship between the flower and fruit. Perhaps the issue that most confounds me in regard to public understanding of plants is the fact that people do not totally get the relationship between flower and fruit. It seems not to be part of our common base of knowledge that the fruit is the fully developed pistil – in essence another stage of floral development. In talking to people, one gets the impression the public believes flowers and fruit are two different things.
The distinctive size, form, and color of a fully mature fruit does make the direct correlation sometimes difficult, in that physical differences between a receptive pistil and the mature fruit can be astonishing. To crystallize this understanding, I attempt to get my hands on as many flowers as possible in order to establish the direct connection. If the season is right, there are often pea or bean flowers available. In spring, Sweet Peas make excellent samples, in that flower parts are large enough to see without handlenses or microscopes. Because I have dissection microscopes available for students, I can also resort to using Clover, which though tiny and difficult to manipulate is almost always in flower. Regardless, any bean/ pea flower will serve to establish the connection between the pistil and the mature bean or pea pod (the legume).
Another easy combination that irrefutably establishes a direct correlation is the flower and mature fruit of Tomato. With vining Cherry Tomatoes so common, one can often find fresh flowers and fruittoexamine. One of the most beautiful comparisons is in citrus. In Southern California gardens, Eureka Lemons bear flowers and fruit much of the year. The pistil is quintessentially textbook – looking just like any student could wish, with a potbellied ovary, an obvious style, and a clearly defined stigma. Establishing the relationship of that pistil to the overblown fruit is ironclad.
Based on our earlier work with the legumes, there are many directions one can take. Because this is a class for gardeners, I trot out fruit commonly grown in gardens, and compare them in groups. The purpose is to make sense out of the structure of different kind of fruit such that students can appreciate how fruit help explain a lot about plant development and relationships. Other major groups to consider include the potato family (Solanaceae), the melons (Cucurbitaceae), the citruses (Rutaceae), and the roses (Rosaceae). For closers, we cover some real curiosities, including artichoke (the daisy family, Asteraceae), Corn (the grass family, Poaceae), and Fig (the mulberry family, Moraceae).
We grow many Solanaceous plants, including Tomatoes (Lycopersicon), Eggplant (Solanum melongena), Peppers (Capsicum), Tomatillos (Physalis), and of course, Potato (Solanum). Tomatoes are among the easiest to understand, because each of the two to several chambers of this berry represents carpels that fused together in the single pistil and now have matured as a multi-carpellate fruit, The fruit of Eggplant, Tomatillo, and Potato resemble each other in that there are no open chambers and everything grows together as a single solid mass. Peppers have their own characteristic fruit, with three or more hollo chambers (carpels) with seed developing basally. This basal placentation is one way you can distinguish the Capsicum Peppers from other members of thefamily.
Getting into the melons, with the exception of Chayote, all of the cucurbits are distinguished by their characteristic 3- carpellate fruit, most readily observable in cross-sections fo cucumber. There are differences in that some, like the gourds and pumpkins, will be hollow, while cucumbers and melons are flesh-filled, but all bear their seed along three placental zones on the inner wall of the fruit (not from a central axis.) This characteristic, called parietal placentation, marks the melons so thoroughly that we even designate them as their own fruit type, the pepo. As has been said, “By their fruits, ye shall know them.”
Another group of plants that make a kind of fruit so characteristic as to merit its own special name would be the many important citruses. Because some botanists like to believe that the Golden Fruit of the Hesperides was likely some relative of the orange, we call the kind of fruit made by citrus a hesperidium. I already mentioned the utility in showing students citrus flowers and mature fruit. This can be amplified by making cross-sections that show the pistil is made of about 10-12 segments – each of which is a carpel. (for a technical article, look on-line in JSTOR for: Carpel Polymorphism in Citrus Fruit, Brent Tisserat, Paul Galletta, and Daniel Jones, 1990, Botanical Gazette 151:54-63) This means each segment represents one of many fertile leaves that grew fused together to make the hesperidium. These fruit have axile placentation, with seed being produced on a placenta in each segment that runs up the central axis. There are a couple of variants that help elucidate the separate origins of these carpels. One is a lemon cultivar called ‘Buddha’s Hand’, in which the segments are imperfectly fused, such that each carpel projects somewhat separately, spiraling to the apex. Another is Australian Finger Lime (Microcitrus australasica), a nifty citrus that produces single carpellate fruit. This plant is worth growing just to share with visitors, who will find it as pungent as kumquats and take great amusement at the caviar-like texture of the juice sacs (trichomes) that line the inside of thefruit.
The last major group to tackle, the Rosaceae, delivers another lesson in plant structureandvariation. Through comparing the fruiting structure of a Rose, to that of an Apple, to a Peach (or Plum), to a Blackberry (or any of the dewberries or Olallieberries) and finally to a Strawberry, we can see how modest alterations in development yield completely different results. I pass out samples of each fruit type, but also make simple diagrams on theblackboard.
We have to begin with the fact that the basic flower for all of these plants is really similar. A species rose normally has five sepals, five petals, numerous stamens, and several separate pistils. The pistils will be in a floral cup formed by the stem and other flower parts. All of the others will also have five sepals, five petals, and numerous stamens. But the apple will differ by having five pistils that are sunken below the other flower parts and completely imbedded in the floral cup. The peaches and plums differ in having a single pistil on the tip of a floral stem that does not make a cup. Blackberries produce an elongated floral stem that bears many pistils lined up in a tight spiral. And Strawberries produce numerous free pistils spiraling around a globular floral stem. In every case the pistil only produces a single seed.
You have to stop here, for a moment, and think how different this flower structure is from others we have discussed. In the beans, the pistil was made of a single segment (carpel). In the Solanaceae, there were a few carpels totally fused to form a single pistil. This was also the case with the melons (with three carpels in a single pistil) and citrus (usually with around 12 carpels comprising the stigma).
Roses, on the other hand, do something that botanists consider a bit old-fashioned (primitive,orancestral). In roses the carpels remain free of one another. They do not fuse together. This is also true of plants in the Magnolia, Laurel, and Ranunculus families. Together with roses, they constitute some of the more ancient groups of dicots.
It is, however, probably a bit novel for most gardeners to think that a single flower could produce more than one fruit.
But for many plant groups, this is normal. A mature pistil is a fruit. If a flower produces several mature pistils (multiple carpels that are not fused together), then technically each is regarded as a separate fruit. This understanding up-ends a simple approach to understanding flowers and fruit. But if you get the rose family under control, you are well on your way to mastering flower and fruit structure across all of the Angiosperms (flowering plants).
In the roses, we have five fruiting configurations produced by flowers that are awfully similar at first glance. Let’s label them within their genera: Rosa (Rose), Malus (Apple), Prunus (Peach Plum, Cherry, Apricot), Rubus (Blackberry, Olallieberry, Dewberry), and Fragaria (Strawberry). Each shows its own characteristic number and positioning of pistils, but when we introduce other variables, the potential changes. Fruit walls can be of different textures and layerings. A one-seeded pistil that develops with a touch wall (such as a sunflower) that does not split open at maturity is called an achene. A one-seeded pistil that forms a hard inner wall and a soft outer wall is called a drupe. Pistils imbedded in flower stem make a pome. We find these three fruit types in this group.
Rose makes big, hairy achenes. Apple forms its seed in a pome. The single pistil of each Peach flower becomes a drupe, as do each of the numerous pistils in Blackberry flowers. The scores of Strawberry pistils develop as small, hardachenes. This means that some things are simply not what they appear to be.
I would think most people would consider a rose hip to be the fruit of a Rose flower, though it is really a fleshy to leathery floral cup inside which there are several single seeded fruit (achenes).
The flower stem of Apple surrounds five or so carpels, making for a firmly succulent pome – in which the fleshy and edible portion is really stem.
The single-seeded Peach fruit (a drupe) has a fleshy outer wall and a very hard, wrinkled inner wall that protects the single seed inside.
If you imagine a stem with twenty very tiny plums attached to it, you would have an approximation of the Blackberry. This is a result from development of the flower stem Blackberry blossom, which formed as an axis covered with tiny pistils, each of which would become a miniature drupe (a drupelet). When you consume a Blackberry, you are really consuming several tiny drupes that developed from a singleflower.
But we reserve Strawberry for the surprise ending. The sweet and fleshy red tissue we enjoy is all stem, while the annoying hard seed-like achenes that get stuck in your teeth are the true fruit here. The scores of tightly-packed pistils formed in each Strawberry flower became tiny, hard achenes that became distanced from each other as the stem fleshed out.
If I have time, and want to risk complete confusion for a bunch of students, I bring out a few really common but baffling fruit. Corn is a must, and can be compared to Pineapple in amusing ways. Artichoke is fun. And then there are Figs.
Considering the fact that Corn is one of the most important crop plants in the world, it is disheartening that people today probably know less about its structure than would someone for whom corn was a food crop a thousand years ago. I work with gardeners; some of whom even grow a few corn plants from time to time. But it always seems to be a wonderful surprise that each kernel of corn has its own dedicated silk.
THAT IS, each corn kernel is a grain, so it is a single-seeded fruit from a separate flower. The style and stigma for this fruit is the silk. This tells us that an “ear” of corn is a stem that is packed tightly with female grass flowers. In modern hybrids, that means each ear of corn has about 500 flowers that will mature as kernels [see Chapter 5,Section5]. It is a marvelous thing, a wonder.
Of course, this also means that each kernel of corn could only develop once a pollen grain landed on its connected silk and grew a tube from the tip of the silk to the base of the pistil.
What is curious to me is that the silks remain attached and alive through much of fruit development, something I cannot explain.
Pineapple is somewhat comparable to the ear of corn, in that each diamond pattern you see on the surface of a pineapple reflects the maturation of a single flower. In this instance, however, the pistil does not turn into a clearly de-marked fruit, rather the pistil and surrounding flower parts develop together, with considerable fleshiness, sealing in the liquid. And each flower melds with its neighbors. You can confirm this by cutting thin-sections of a pineapple, where you can detect the mature pistil and other fleshy flower components in which it is imbedded. Most people know that if you cut off the tuft of leaves on top of the pineapple, that can be rooted to grow another pineapple plant.
This means there is a growing tip in that leaf cluster, which makes me think of pineapples in a completely distorted way. I imagine each pineapple as one module in some self- replicating stacking game. Picture the pineapple that begins the stack, and visualize mature foliage spiraling out off of the stem on top. The growing tip that created this foliage then produces a segment of bare stem just tall enough to clear the leaves after which it produces a segment of stem bearing spirals of flowers – which will become the fruit. This means that when the stem tip is through with its run of flowers, it goes back to the business of making leaves (stunted leaves).
So you get back the marketable thing we call a pineapple. Once fruiting is over, the tip returns to the task of producing full-sized leaves (which can be about 2 feet long). With that task complete, and enough leaf surface created to support flowering and fruiting, the tip makes another clear segment of bare stem after which it returns to its task of making flowers.
What we are observing is a stem tip that’s in some kind of iterative loop, with three personalities. It makes leaves, then stalk, then flowers, then leaves, stalk, flowers, etc, reminding me of some episode of Star Trek I saw a few decades back.
Thinking about this a bit more, we know that in the axil at the base of each green leaf is a tiny bud that remains dormant.
Subtending each flower (which started as a bud) is a leaf that is highly reduced and therefore called a bract. The base of the bract turns fleshy along with the pistil and other flower components to yield the mature thing we call a pineapple. It seems the genetic instructions are straightforward, and something pretty clearcut happens that turns each format on and off…..
The stem itself remains reasonably constant in size through these phases. It is the inedible core of the pineapple and the practical reason processors had to sell us all on the fact that pineapple slices come to us with a hole in the center.
Though the stem of pineapple is way too tough and fibrous to be edible (it is one of those monocots), artichoke stems are
| Mulberry is also a fig, and lays out flowers along a stem, flowers that mature into something that looks a lot like a Blackberry. But completely unlike Blackberry, each fleshy fruitlet (a drupelet) was produced by a differentflower. |
about the only edible component of this flowering head.
The construction of the Pineapple gives us something that contrasts nicely with that of the Artichoke. Both are flowering stems, but the Pineapple stem is indeterminate, which means the growing tip does not terminate with flowers. In the Artichoke, however, the growing tip winds itself down with flowering and ends as a predictable head of flowers, as is true for all of the daisies.
With the Pineapple, each flower is subtended by a large bract, while in the Artichoke and other daisies the stem creates a series of large bracts that are sterile, after which it comes to the task of making a disc of tightly packed flowers that each have their own much smaller (but still annoying) bracts. The larger bracts encase the disc of flower buds until flowering time approaches. This means the Pineapple is harvested after flowering, because we plan to consume the mature fruit (and everything else associated with it).
Our intentions are very different for the Artichoke. We have little interest in the flower or the fruit, because the artichoke is pretty much a vegetable. People clean out the flower buds (and bracts) as the messy and inedible “choke”, rendering the tender supporting stem available as an Artichoke “heart”. I guess we also consume some leaf tissue, because an appetizer activity involves breaking off the cooked bracts and dipping them in butter or dressing in order to tease off a bit of pulp from the surface. But we all know there is not much food value in thatpulp.
Imagining a mature Artichoke stem, topped with flower buds, you have a reasonable model for the structure of Dorstenia, a greenhouse weed we call Exploded Fig. We call it a fig because it’s in the fig family (Moraceae), which means that Dorstenia is not at all related to the daisies. But it produces a packed disc of flowers on top of a flattened stem, which is reasonably comparable to what daisies do. But Dorstenia is a weirdo, other figs behave differently.
That does not mean other figs fail to behave weirdly. When you grow edible figs, Ficus carica, there is little else in your garden with which to draw comparisons.
Figs are the most contemporary and bashful products of garden and orchard. They make flowers, but you will see no evidence of that. Even if you do seek out the flowers with the aid of microscopes, there is little to crow over. Fig flowers are highly reduced, down to the business end, producing only pistils and stamens. But those flowers are hardly visible because they are tucked completely inside the end of a stem, like a Dorstenia that has been folded back on itself such that all of the flowers are enclosed by stem.
Fig growers know there will be a crop when small knobs appear in the new growth. Each knob is an inflorescence (called a syngonium) that bears the smallest portal at its tip. Through chemical invitation, tiny wasps enter the tip, where they lay eggs. Their hatched and mature progeny leave the comfort of the syngonium covered with pollen, which they carry to another inflorescence where pollination is achieved and the progeny become entombed by the growing fig. It is interesting to think that instead of calling the fig a syngonium, we could have called it a sarcophagus.