By Terry Ashley
You don’t have to be Sherlock Holmes to conclude that plants are different from animals. Animals travel: they swim, fly, crawl, climb, run. Plants don’t, yet they occupy as many different habitats as animals. Most animals adapt to extreme changes in temperature by changing their environment. Plants don’t have that luxury and so have several astounding adaptations at their disposal to help them survive changing weather conditions.
Look at trees in winter, and your first observation might be how bare they are; but notice the trees that haven’t lost their leaves. First consider the conifers: the pines, the cedars, the firs, and spruce, all with needles that are waxy and thick. Wax and water (and snow) don’t mix. Snow slides off. And conifers have another trick up their limbs. The structure of xylem cells in their wood is springy, allowing their branches to bend under a snow load and let it slide off.
Many broadleaf trees and bushes, including holly, mountain laurel, and rhododendron, also keep their leaves, which are thick and waxy. Holly leaves are convex — a miniature upside-down sled, a waxed incline. Their wood is extraordinarily tight-grained and strong, so it can usually handle a snow load. Rhododendron leaves curl up when cold, and you can read the temperature by observing how tightly they are curled. Couple that with their coating and the vertical incline when curled — snow, good luck sticking on that.
All biological processes have an optimal temperature for activity. If it’s too cold, photosynthesis grinds to a halt, and the plant’s entire metabolism slows to accommodate the weather. But conifers’ thick needles allow them to keep photosynthesizing on sunny days. Those trees that lose their leaves can’t keep photosynthesizing. Hanging on to their leaves is a liability. When snow falls it sticks. The composition of their wood is brittle, and the limbs break. Better to shed leaves than endanger the tree.
Plants also have methods to protect against wind, which can uproot them and dry out their leaves. For trees and shrubs, the shapes and irregular contours of their leaves slow and alter the flow of air. It’s friction: think miniature brakes, but brakes braking in all directions. This creates turbulence and slows the flow. Objects near the ground also act as brakes. Moss with its tiny scaly leaflets can disrupt the flow. So can moss-sized plants such as partridge berry, rattlesnake plantain, pipsissewa, and tiny dandelion-like composites. All of these have waxy leaves and their own means of disrupting air flow. Galax is larger than most of these, but its edges are ruffled — a great disrupter. Partridge berry leaves are tiny so each one is a disrupter. The prominent veins of rattlesnake plantain and pipsissewa create a rough surface. But the little dandelion-like composite is especially fascinating. Not only does it have irregularly shaped leaf edges, but it has “hairs,” each a tiny disrupter.
Red leaves are evidence of another tool in the arsenal of some plants. Anthocyanins, which produce these brilliant colors, are red/blue soluble plant pigments that are contained in vacuoles (membrane bound vessels) within plant cells. In summer they give color to flowers and fruits, but in autumn they take on another role. Metabolically, anthocyanins are in a complex sugar pathway that among other functions helps protect against cold damage. The color red absorbs more heat than green, so raises the temperature of the leaves and prolongs photosynthesis. Anthocyanins can also convert light energy to chemical energy, again allowing continuation of photosynthesis. So red leaves, whether on a tree or an inch-high herbaceous plant, are a key component to plant adaption to cold weather.
Many plants plan far in advance of the coming winter. Forest spring bloomers flower before the trees leaf out. While they bloom, their leaves are busy photosynthesizing before the canopy closes darkening the understory and slowing or halting their activity. As their leaves die back, the nutrients for next year’s growth are already stored in their rhizomes or bulbs safely underground. The bulbs aren’t really “hibernating,” however, they are busy producing next year’s flowers. They are slowly undergoing meiosis, the dual cell division that eventually makes the pollen and ovules that lead to the next generation and other parts of the flowers. Some bulbs even require low temperatures to bloom.
All of these tools and adaptations are evidence of a type of intelligence we often don’t value enough or even recognize. Plants have been here long before us and will remain long after we’re gone. They are essential for all life, and humans can take a lesson from their ability to adapt to environmental changes. Not only do they sustain human life by filtering our air and providing medicine, but they also add beauty and wonder everywhere we look.
Terry Ashley has always been fascinated by plants. From a young age she was asking questions like, “Why aren’t Indian Pipes green?” She earned a bachelor’s degree in botany from Duke University, where her favorite course was Plant Anatomy. There she began to learn how plant cells hold the key to how plants adapt to their environment. She earned a Ph.D. in genetics from Florida State University, and followed a career in the microscopic study of the cells that produce mammalian sperm, at Oak Ridge National Laboratory and the Department of Genetics at the Yale School of Medicine. A Gainesville, FL, native and accomplished fine arts photographer, she loves to take a picture of a plant and combine it with a microscopic image of cells of the same species. In retirement, she continues to ask questions, with a current focus on rare and endangered plants and factors that lead to their survival/extinction.