272 CAUSES OP DEGENERATIVE EVOLUTION 



the function of the calyx has been reduced, or has 

 disappeared on account of the transference of its 

 function. 



(6) Atrophy of roots. Eoots in most plants per- 

 form two functions : they fix the plant in the soil, 

 and, chiefly by means of the delicate hairs on their 

 youngest parts,- they absorb water and dissolved 

 mineral substances. Sometimes, however, instead 

 of having root-hairs, the rootlets enter into a 

 kind of partnership with a fungus, which lives 

 in their tissues, and absorbs by its processes the 

 necessary food materials from the soil. The pine 

 and beech are examples of this. 



In some orchids (CorallorJiyza, Myrmechis) the 

 fungi lodge in the subterranean part of the plant, 

 and the branches, having no function, disappear. 



In most of the epiphytic Bromeliacese the roots 

 are useful only to anchor the plants ; the absorption 

 of water takes place entirely through new organs 

 developed from the leaves. The roots are few in 

 number and small, and after the plant has obtained 

 a firm position their growth almost ceases. In 

 another species (Tillandsia usneo'ides) of the same 

 family the roots have completely disappeared. 1 

 This plant, called by the Brazilians, " the plant of 

 the air," fixes itself to branches of trees by its 

 leaves, and is easily transported by the wind. Its 



1 For further details concerning the Bromeliacese see Schimper, 

 Die epiphytische Vegetation Amerikas, in Schimper' s Botanische 

 Mittheilungen aus den Tropen. Jena, 1891. 



