No better idea of the present size and importance of these two groups of plants can be gained than to state the fact that perhaps not over 500 different kinds of gymnosperms, all of which are trees and shrubs, are known. All the rest of the flowering plants in the world, comprising over 150,000 different kinds of herbs, shrubs, and trees of infinite variety, are angiosperms and therefore bear ovules in an ovary, followed by seeds in or on some sort of a fruit. It would almost seem as though the simplest way to dispose of this great mass of plants would be to sort them into trees, shrubs, and herbs. For all of them belong to one of these types of plant growth, and the ancient students of plants, just before the time of Christ, actually divided all flowering plants into these three classes. This, of course, threw the coniferous trees in with all other kinds and, as we have already seen, they differ from all other kinds in the important character of having naked ovules.
Here, again, in order to get some system out of apparent chaos, we must fall back on some fundamental character. And, again, it is the product of the reproductive process in all this host of angiosperms which furnishes the clue. In the seeds of many of them the young embryo has folded up within it two seed leaves, while in all the rest only one. As we saw in Chapter I, these seeds germinate either with a single seed leaf, like corn ([Figure 85]), or with two seed leaves, like beans ([Figure 81]). Every one of these angiosperms belongs to one of these classes or the other, and perhaps more extraordinary still is the fact that in those with one seed leaf there are associated certain leaf and flower characters, while those with two seed leaves are always very different.
In the monocotyledons, or plants with a single seed leaf, the leaves are practically always parallel veined ([Figure 83]), like corn and grass, and lilies and palms, and hundreds of others. Also, they nearly always have the parts of their flowers in threes ([Figure 84]). That is, they have three sepals, petals, stamens, and often pistils, or multiples of three. The common trillium or wake-robin, for instance, has three sepals, three petals, six stamens, and three styles. With a few exceptions, and nature seems to delight in producing a few such, all monocotyledons have this parallel-veined leaf character and flower parts in threes or multiples of three.
Plants which send up two seed leaves ([Figure 81]), on the other hand, bear practically always netted-veined leaves ([Figure 79]), and the parts of their flowers are nearly always in fours or fives or multiples of these numbers ([Figure 80]). The well-known wild geranium has five sepals, five petals, ten stamens, and a five-lobed or five-celled ovary. There is some individual variation from this plan, sometimes one organ and sometimes another having more or less than the regular number. But so overwhelmingly true are these distinctions that dicotyledons, or plants with two seed leaves, and monocotyledons,
Dicotyledonous and Monocotyledonous growth habits contrasted. Figs. 78-81. The trunk of a dicotyledonous tree showing division of the wood into heartwood, sapwood, and cambium, which the removal of a piece of outer bark exposes. Note the net-veined leaf (79), the seedling with two seed leaves (81), and with the parts of the flower in 5’s (80). Figs. 82-85. Monocotyledonous plant. Note the lack of zones of wood, cambium and corky bark. Such plants have parallel-veined leaves (83), parts of their flowers in 3’s or 6’s (84), and germinate with a single seed leaf (85).