274 CAUSES OF DEGENERATIVE EVOLUTION 



functions of leaves, while in Tillandsia usneo'ides 

 the leaves have taken on the functions of roots. . 



The Podostemacea3 are aquatic plants which live 

 in warm regions attached to rocks in cascades. 

 The stems and leaves are completely absent, the 

 flowers even being produced on the roots. Some 

 of the roots become closely attached to the stones. 

 Others which are green and ribbon-like, float in the 

 stream, and serve for assimilation. 



3. Atrophy from lack of nutrition. 



In some cases of degeneration, the organs do not 

 lose their functions, but become reduced, merely 

 because adjacent parts rob them of their nourish- 

 ment. This kind of lack of nutrition which results 

 in local atrophy, must be distinguished from the 

 general limitation of food-supply which is the 

 ultimate cause of all degeneration. The possible 

 amount of food within the reach of any organism 

 is limited, but besides this, the share of the absorbed 

 food obtained by any particular organ or part of 

 an organ may be limited with a resulting atrophy 

 of that organ or part. Atrophy of this kind may 

 be accidental or normal. 



1. Parasitic castration. A good instance of acci- 

 dental atrophy of this kind is found in Melandrynm 

 album where the ravages of a fungus Ustilago 

 antheridarum may produce parasitic castration. 1 



1 See Giard, La Castration parasitaire in the Bull. Scient. de la 

 Frame et de la Belgique. 1887, 1888, 1889. 



