Botany transcends the borders of time and space. Past and present are welded together in the body of the plant and distance, certainly national borders, is hardly an issue in plant biology. Modern plant form, the most obvious feature of plants we see all around us, goes back hundreds of millions of years. We detect evidence of ancient morphological features in every plant we observe, and we can infer evolutionary change in a host of plant features. Luckily, plants are there for us to observe on the cheap, without specialized laboratories, costly expeditions, or elaborate tools. Perhaps more than any other organism, plants tell a story of their long evolutionary history in their shape. Their frank physical form-- roots, shoots, leaves, flowers, and seeds, tells us about how plants have adapted to their surroundings over millions of years of evolution. By observing plant features we can look deep into the past of the species. Their structural adaptations are written, or more accurately, sculpted all over them, for example in the shape of leaves and flowers, in the modular growth form that every plant possesses, and in the many ways that plants maximize photosynthesis while minimizing water loss. And those are just macroscopic features. Even the molecular evolution of plants is apparent just by using our noses. The way a lilac smells, the flammable essential oils of an orange peel, or the acrid odor of the tree of heaven, Ailanthus altissima, all provide insights into plant evolution. You don't need to be a scientist to cross the border between plants and humans, because so much of their evolutionary history is there to observe.
The bottom line of plant evolution is that plants have learned to cope with a hostile environment since they first moved onto land. They are especially adept at preserving resources such as nutrients and water. There are two main reasons for this. First, plants are rooted in place. Their roots can spread toward nutrients or they may become symbiotically involved with fungi or bacteria to enhance their nutrients, but plants cannot get up and search for a more favorable spot. Second, plants lose water as they perform photosynthesis. As we'll discuss in detail below, the photosynthetic process involves a constant exchange of precious water vapor for abundant carbon dioxide, which plants use to produce carbohydrates. To cope with the constant threat of dehydration plants have devised thousands of strategies for conserving water, some of which we'll study in these pages. As experts at adaptation, plants offer us valuable lessons about how to cope and thrive in our surroundings, how to build better buildings, plant better green spaces, and how to insulate ourselves from excessive cold and heat. Many of these solutions are quite simple, just waiting to be adapted by designers. So whether we consider the succulent leaves of a beach plant, the prickles of a cactus, or the climbing habit of a jungle vine, the basics of plant form open a useful window on plant evolution.
Most of the plant form on these pages reflects growth patterns of seed plants, angiosperms (plants with flowers) or gymnosperms (plants like pine trees). These plants are the most common and therefore familiar, but seedless plants such as ferns have much to offer as well. It is interesting to note that certain growth patterns of fern forebears can be detected in modern plants, and these are well worth studying. For example, traces of the way fern fronds unfold, the wonderfully linear display of spore cases on the undersides of fronds, and the free expression of hairy surface growth, are all features that can be observed in modern land plants. Ferns are pretty as well. Their delicate beauty, gently unfolding above the fallen leaves of last autumn, belies an era when ferns were the dominant land plant, producing forests of towering trees.
Spore-bearing ferns and their allies are wonderful in their own right. They carry rich information about the evolution of plant form over millions of years, but seeds, which we will examine throughout these pages, are in many respects the signal example of plant evolution. Their role as dispersers, protectors, and nutrient providers are the key to understanding the overwhelming success of plants on Earth. Seeds with their many adaptations to life on land help us understand why plants are so widespread in the terrestrial environment. Because seeds are so well built for dispersal and so efficient in the other jobs they do, such as storing and mobilizing nutrients for the developing pre-photosynthetic embryo, they have helped plants colonize all parts of the planet. Plants transcend physical borders thanks largely to seeds, a radical evolutionary innovation.