5. The seeds are always enclosed within the carpels, and have generally two seed-coats.
6. Within the seed are always either two cotyledons, as in the bean, or one cotyledon, as in the grasses. Thus when the seedling grows out of the seed it may have two first leaves or one only.
These are the chief characters of the whole big family of the flowering plants, but this big family is separated into two smaller groups according to the number of cotyledons in the seed. Those that have two form the group of Dicotyledons, those with one the group of Monocotyledons. This may not seem a very important point to form the ground for separating plants with flowers so alike as tulips and roses, but we find that, as well as the number of cotyledons, many other differences distinguish the two groups when we separate them in this way. For example, the Dicotyledons have the veins of their leaves so arranged as to form a network, as in the lime, while the Monocotyledons have them parallel, as we noticed in the grasses and lilies.
We also find that it is only in the Dicotyledons that the plants have rings of wood in their stems, as is the case in the lime, oak, and many others.
In the numbers of the parts of the flower, we also find differences between the two groups; for example, the Dicotyledons have generally two, four or five, or a multiple of these numbers such as ten, as we see in the poppy, primrose, rose, and many others; while the Monocotyledons have the parts of their flowers in three or multiples of three, as in the lily, tulip, and daffodil.
These differences between the Monocotyledons and Dicotyledons, however, are not nearly so important as their likenesses, for they agree in the main points (1) to (6), and therefore belong equally to the great family of the flowering plants, which is the most important family now living.
CHAPTER XXIV.
THE PINE-TREE FAMILY
Since trees such as the oak, beech, and lime all belong to the family of flowering plants, you may be surprised to find that the pine-trees are separated from them. Yet all the trees like pines, Christmas trees, larches, and many others, form a family of their own. You will see why this is, if you look at a pine-tree carefully, and compare its characters with those we saw in the flowering family. In the first points the two families are alike.