Every spring, gardeners go out to plant. We prepare the soil and carefully bury a tiny seed. We might water, if the soil is dry. Mostly, we watch and wait. We fully expect that seed to germinate and grow to maturity. But what is actually happening beneath the warming soil? What is a seed, anyway? How does it know when to break dormancy and germinate? How does it know which way to grow? Since spring is approaching, I thought I’d write a series of posts on seeds—where they come from, what they are, what happens to make them grow. First, we need a diagram of a basic flower. I like this one, from the University of Illinois Extension. You’ll need to refer back to this as we go along.Until I took a botany class, I always assumed seeds were the plant counterpart to eggs. Just as chickens lay eggs that turn into chicks, I thought that plants produce seeds that turn into sprouts. Was I ever wrong!
Plants do have their own version of eggs and sperm, but that’s not what seeds are. Rather, plants have ovules and pollen. Ovules reside in the ovary, found at the bottom of the pistil, at the base of a flower. They correspond to eggs.
Pollen, the plant version of sperm, is produced on stamen, the (often yellow) enlarged tips of the stems inside a blossom. Those stems are called filaments—biologists love to give names to absolutely everything. The stamen and filaments together make up the anther. You can easily see the anthers, with their yellow stamen, hanging down from this columbine blossom.
Many flowers have more complicated variations on this theme. Sunflowers, for instance, are a composite of many flowers fused into one giant blossom.
Not all flowers have all these parts. Some kinds of plants produce both ovules and pollen in every flower. Others have male and flower on the same plant (termed “monoecious”), but each individual flower is either male or female. Still others produce either male or female flowers, but not both on the same plant (dioecious). In other words, each plant is either a girl or a boy. Let me give you some familiar examples.
Poppy flowers have both male and female parts. Here you can see the stamen surrounding the pistil (with a large, sticky stigma ready to catch some pollen.) The large ovary at the pistil’s base will eventually grow into a seed pod. It is technically possible for a single poppy blossom to produce seeds. (However, many plants have ingenious ways of preventing self-fertilization, thereby ensuring a mixing of genes.)
The flowers of cucumbers, zucchini, and other members of the squash are either male or female, and both types of flowers appear on the same plant. The male flower is on the left.
Breeders have messed around with this trait, so now you can buy cucumbers that only have female flowers. A few “normal” plants are included in a seed mix to ensure pollination. Having mostly female flowers means you get more cukes from your garden.
Holly plants are dioecious—they are either male or female, but not both. If you want holly berries, you most plant a female shrub. Additionally, there must be a male nearby to provide pollen. The berries won’t grow unless the pollen from a male plant must be transferred to the pistil (and from there to the ovules) of a female plant. This is called pollination.
Some other dioecious plants are junipers (shown below; the male is on the left), kiwi vines, willows and aspen. “Fruitless Mulberries” are fruitless because the nurseries sell only male trees. Unfortunately, the pollen produced can trigger allergies in many people (including me).
Female flowers must be pollinated in order to produce seeds. Sneaky botanists have taken advantage of this. You can buy tomato “set spray”—a hormone that fools the flower into believing it has been pollinated, when in reality it hasn’t. The tomato that results from that flower will be seedless—bad for the tomato, but nice for the person eating it!
Next month: How a seed forms.