H22 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 jjlant 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 tlie same plant in its infanc}' much as do the higher 

 plants from the lower. But in spite of the progressive differ- 

 entiation shown l)y 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 hke 

 them anrl 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 essentiallj^ 

 the same in important particulars as the absorjjing 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 



