18.13–18.16: Plants have two types of growth, usually enabling lifelong increases in length and thickness.

Trees grow along a canal in the Netherlands.

With the seed, plants have created an impressive little time capsule. In suspended animation, the embryo patiently waits until the conditions for its new life are just right and then bursts forth with growth. Let’s examine what makes up a seed and how it emerges from its dormancy and begins life.

The seed does not start to grow until the water, temperature, and oxygen conditions are good for germination and growth (FIGURE 18-25). Sometimes this means waiting a matter of days or weeks before germination. For some species, though, the seeds remain dormant even in the face of ideal environmental conditions. Their seeds require something more before they will germinate. They may require passage through the gut of a bird or mammal to roughen up and weaken the seed coat. Or they may require exposure to fire. Such seeds can remain dormant for dozens or even hundreds of years before finally germinating. Conversely, over the course of many generations of plant domestication, plant breeders are likely to have selected the seeds that germinate most quickly and have the fastest initial growth. These features may be optimal under agricultural conditions, where fields are cleared, pests are removed, and competition is limited.

Figure 18.25: The emerging plant.

There can be benefits to producing seeds with coats that require extra “processing” before they can germinate. (1) The need for extra processing can prevent the germination of seeds that have not been dispersed widely enough. (2) The need to pass through an animal gut can ensure that seeds are always left in a dollop of natural fertilizer. Or (3) exposure to fire may ensure that the seed germinates in clearings where competing trees have been burned down and light is available.

Such requirements, though, can also precariously link the evolutionary success of the plant to the behavior of another species or to specific ecological conditions. For example, some of the plants producing seeds that require the extreme heat or smoke from a fire to induce germination are in trouble. As humans have reduced the incidence of wildfires, so, too, have we reduced the ability of many plants to germinate.

The whole purpose of a seed is to package an embryo with a ready-made supply of nutrients and energy so that it has the fuel to start growing before it can photosynthesize for itself. That high-energy package, of course, is exactly what animals are looking for in a food source, too. Consequently, seeds can be a valuable snack, rich in calories. They are also flavorful when ground up, producing spices.

On a somewhat minor note, a confusing issue—though not very important biologically—is the question, “Is a seed a nut?” In short, botanists have given the word “nut” a very specific and technical definition. (It is one particular type of fruit having a hard shell with no suture lines, or “seams,” and with a single seed inside. An acorn is an example of a nut.) The culinary definition of a nut, however, is much less restrictive and encompasses most seeds. By this broader definition, virtually every seed that we eat is considered a “nut,” including peanuts, almonds, cashews, and macadamia nuts—even though none of these would be classified as a nut by the botanist. The whole fruitless debate is a consequence of the common problem of words (such as “theory” or “evolution” or “altruism”) having narrow, specific meanings in scientific disciplines but broader—perhaps sloppier, but usually more widely known—definitions in the everyday world.

But let’s return to the seed, ready to begin its growth. With the proper processing and sufficient warmth, oxygen, and water, germination can finally begin. As germination begins, water is absorbed, causing the embryo to enlarge and the seed coat to burst. Utilizing the fat and starch reserves of the endosperm and embryo, metabolic activity increases dramatically and the plant begins growing (FIGURE 18-26). It first sends down a root and then sends up the shoot. In many plants, the cotyledons can photosynthesize after their energy reserves are used, providing even more energy for the plant. This early seedling growth begins in the apical meristems, which we explore in Section 18-15.

Figure 18.26: From seed to seedling: the initial growth of a plant.