STORAGE STEMS 53 



root-stock is much shortened and vertical in position in other 

 words, merely an extremely short stem it becomes a corm. 



112. Many plants inhabiting semi-arid regions are adapted 

 to the recurring long periods of drought following the brief 

 periods of rainfall by the habit which they have assumed of 

 storing up water. The stems form the reservoirs and are con- 

 sequently of much greater bulk than the other functions of the 

 stem would demand. 



113. A less common type of modified stem is one in which 

 the branch takes up the functions of the leaf. In this case the 

 branch may become flattented and like the leaf in other respects. 

 Thorns in certain cases are also modified branches. 



Modified Leaves 



114. Besides the endless diversity of form assumed by foliage 

 leaves, there are also a number of leaf types in which the function 

 of photosynthesis has been entirely lost. Such, for example, 

 are the bud scale-leaves, which serve as protective organs, and 

 the scale-leaves of underground stems, which are functionless 

 rudiments. Certain kinds of thorns and tendrils, are modified 

 leaves or parts of leaves. Even the function of food storage 

 is sometimes assumed by leaves. The blade of the leaf in one 

 group of plants is entirely wanting and its function is performed 

 by the petiole, which is flattened laterally and has the appear- 

 ance of a leaf blade turned into the vertical plane. 



Homology of the Flower 



115. While the modification of the type forms mentioned 

 in the preceding paragraphs are all quiet common, still they are 

 in every case limited to a small minority of plant species. There 

 is, however, a most important and interesting kind of modi- 

 fication which is practically universal among seed-bearing plants. 



