With a bit of green literacy, you can read a plant like you read a book. How? – the author wants you to ask. What is the language, and where are the messages? Read further for the answers to those questions.
Then, you might also wonder: Why should I read a plant? I script you again, and provide the answer: “A plant can tell you something about where it thrives and what you can know about its life’s work.”
If that’s an engaging proposition, then follow along. With this ten minute lesson in Conversational Botany, you can amaze your partner, astonish friends, and amuse yourself through familiarity with the language of plants. Just give me those few minutes of your time and attention.
Books have leaves; plants have leaves.
Authors compose and publish stories and information, artists create illustrations to convey information and emotion. Plant leaves show form, texture, and dimensions that tell the stories of their lives; what you see is worth a thousand words. And I’m not foisting a foreign language on you; life’s everyday experience is our Rosetta stone.
Let’s review some points that will be no surprise….
- Different kinds of plants produce leaves of predictable sizes and shapes. I hope this is not astonishing. One red cabbage leaf looks pretty much like another. If you’ve seen one ginkgo leaf, you’ve see most ginkgo leaves.
- Leaves wilt when they lack water. That’s because plants transpire, i.e. water evaporates from surfaces, mostly through special pores (stomata) most common in leaves.*
- We associate large leaves with tropical plants. Of course I’m thinking of plants from the wet tropics, which often have enormous, flexible leaves. Conversely, we don’t think of desert plants as being especially leafy. Even when they have foliage, plants native to dry areas typically make hardened, waxy or hairy, often thick or succulent leaves. I don’t guarantee this a hundred percent, but it’s the common condition. Compare plants in the Huntington Jungle Garden with those in the Desert Garden; it’s a straightforward observation.
These simple associations mean that by examining leaves, even knowing nothing else about the plant in question, a gardener or an aspiring naturalist can make reasonable assumptions about that plant’s natural growing conditions, its native habitat. You can read the leaves of plant like a book.
So let’s start with some obvious examples, not really worrying much up front as to what is a leaf versus what is not a leaf. We’ll come back to that following the examples.
I cannot predict that any plant with small or highly succulent leaves is a drylands plant (we call plants that thrive in dry climates xeriphytes). But reading the characters of the leaves and pairing those with other clues the plant offers allows reasonable conjecture. Tough, well-protected (waxes, hairs), smaller, sometimes succulent leaves are associated with plants from dry habitats because those characteristics tend to help reduce water loss. Light is plentiful, water is rare. Large, even floppy, soft leaves tell us the plant can afford to transpire water in exchange for expanded leaf surface. In tropical wet forests, water is not limiting, so greater leaf surface simply allows for more photosynthesis.
To avoid confusion, I should point out that many plants trick the eye. Cacti and a lot of the Euphorbias lack green leaves, having relegated the task of photosynthesis to perennially-green stems. In desert plants, we call these plants “stem” succulents. Once you examine the structure and determine there are no leaves (or, as in Cacti, the spines are its leaves), then you are on a short track to thinking stem succulence is one adaptation that helps the plant cope with water stress. We even see epiphytes in tropical wet forest adopting this and other xeriphytic strategies.
At this point, we have run into a technicality to be deferred for a future posting. But in short, Botanists have standards. A “leaf” is a particular kind of structure plants make, defined by how it is produced by the growing tip and the resulting internal anatomy. Since Botanists group all plant parts as being either stem, leaf, or root, in our world it is unequivocal – being flat and green does not, necessarily, make something a leaf. You’ll find fascinating plants with flat, green stems, and even a few with flat, green roots…. Hang onto those thoughts for the future.
Getting back to reading the leaves, we arrive at a very simple and life-sustaining reality. Green plants grow on their own when given the right balance of Earth (soil & nutrients), Air (carbon dioxide and oxygen), Fire (sunlight), and Water. Different habitats can be understood simply by knowing which resource is the most limiting. We define deserts as places where the limiting factor is water. In tropical wet forests, nutrients are limiting overall, and sunlight can be limiting for understory plants. In wetlands, oxygen and air exchange can be the most limiting factor. In the Arctic, less heat from sunlight changes the climate, while frozen water limits that resource and challenges plant cells with perilous ice crystals.
These honest and easily understood realities are the basis for studies in plant ecology, a field that began with the work of Alexander von Humboldt and brought new awareness of nature that fed into America’s transcendental movement.
“But as this fugitive sunlight
Arrested & fixed
And with the primal atoms mixed
Is plant & man & rock
So a fleeing thought
Taken up in act & wrought
Makes the air & the sun
And hurls new systems out to run”
― Ralph Waldo Emerson
*This cuts two ways. If plants lose too much water, the cells will burn or collapse and the plant will suffer. If water does not move through plants, moisture and nutrients will fail to move from the soil to other parts. Without evaporation, there would be no upward flow and no cooling of foliage. Some plants would roast on hot days. Once again, this should be no surprise. Humans have similar issues of their own – blood flow, sweating and voiding waste are crucial to your well-being, but too much loss can be deadly. There is a wonderful word for the right balance in humans and plants – homeostasis.
“How can I stand on the ground every day and not feel its power? How can I live my life stepping on this stuff and not wonder at it?” — William Bryant Logan, Dirt – The Ecstatic Skin of the Earth
PS – Books have spines and cacti have spines. The spine of a book is the midrib along which the leaves fold, the edge that holds everything together. In a real way, you can think of a cactus spine similarly, as the spine of the leaf, its midrib (main vein), stiff and pointed, lacking the leaf blade. It doesn’t take much for a plant to create a spine out of initials (these are beginning cells, the ones that initiate formation of an organ or structure). There is, of course, no homology – no parallel origin. It is just plenty curious that you can imagine how a spine is a specialized leaf by imagining what would be left of a book if you removed its pages.