MODIFICATION OF THE ORGANS OF PLANTS 69 



These foliage leaves, which are solely assimilative, 

 are philogenetically the oldest, for the assimilative 

 function of leaves was certainly an earlier function 

 than any they may now exercise ; but it is not un- 

 common to find both basilar (Niederblatter) and 

 apical leaves (Hochbldtter) as well as foliage leaves 

 (Laulibldtter) in the same plant, and even growing 

 from the same branch. These leaves, which arise 

 directly from the foliage leaves, have more or less 

 completely lost their primitive function, and have 

 assumed others. Their structure has undergone 

 corresponding modifications which are all attended 



for some time without any evidences of differentiation, and con- 

 stitutes what is called the primordial leaf. Next the leaf is 

 differentiated into a proximal part, which almost surrounds the 

 stem (the hypopod), a distal part (the epipod), and an inter- 

 mediate part (the mesopod). From this point the various parts 

 of the leaf begin to develop individually. 



Little foliacious lamina (the stipules) frequently arise laterally 

 from the hypopod, and, while still in the bud these stipules, being 

 often much larger than the rest of the leaf, serve as a very effective 

 protection to the young organs. 



The way in which the epipod differentiates varies very much in 

 different cases. The lamina is developed from it, and little buds 

 are gradually formed around the point until the whole is ready to 

 branch out. At this stage the young leaf is still rolled up within 

 itself, and protected by stipules, when there are any. 



Next, the mesopod grows into the petiole, and the growth of the 

 petiole causes the separation of the different parts of the leaf. 



The leaves of many plants are much less complicated than 

 these. Sometimes the mesopod is missing, and the hypopod and 

 the epipod are left in contact. Sometimes there is practically no 

 hypopod, and in some cases the primordial leaf develops with- 

 out any differentiation into hypopod and epipod. See fig. 49 

 (Sempervivum arachtwideum). 



