322 THE PARTS OF A SEED-PLANT 
flower); and of dissemination (the fruit). The first three 
of these groups, since they have to do primarily with the 
individual life of the plant, form what is called the vegetative 
system, while the others being concerned only with propaga- 
tion and the care of offspring constitute the reproductive 
system. 
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 differ- 
entiation 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 essen- 
tially alike as holding corresponding places in the funda- 
mental 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 
ee 
