Part 97. Morphological differentiation. From what has been said of the life history of flax it is plain that the differentiation of its parts progresses as the plant grows older. We saw that the parts of the embryo within the seed are all much alike, as are also the young foliage leaves and floral organs within the bud; but as the plant matures and its needs become more varied the parts come to have different functions to perform and take on the various forms which fit them for their special kinds of work. Thus, the mature flax differs, from the same plant in its infancy much as do the higher plants from the lower. But in spite of the progressive differentiation shown by a growing plant we feel that even its more highly specialized organs correspond somehow in a fundamental way with certain of the earlier or less specialized ones. Petals, for example, although widely different from cotyledons in function, are yet in some ways so much like them and like ordinary foliage leaves that cotyledons are often called "seed-leaves" while petals are familiarly known as "leaves of the flower." So, too, in comparing the parts of different plants we often find a fundamental likeness along with marked differences in function. Thus, the climbing roots of the ivy before mentioned are essentially the same in important particulars as the absorbing roots of flax.

Not only among plants but also among animals it is true that analogous organs may show important differences, and similarly that organs which are not analogous may be essentially alike as holding corresponding places in the fundamental plan of structure. A man's arm viewed as an organ for grasping is plainly the analogue of an elephant's trunk, and an opossum's tail; while viewed as a member of the body it corresponds to the fore leg of a horse, the flipper of a whale, and the wing of an eagle. Considerations of this nature lead us to inquire; What is the fundamental plan of structure exhibited by our typical plant? and What may we rightly regard as the members of such a plant-body?