DESERT PLANTS 



545 



ures from the prototypes, that the relationship of the group is very 

 difficult to determine. 



Chief interest in the present connection lies in the fact that in the 

 evolutionary movement the members of the group have undergone all of 

 the specializations of the spinose forms in addition to a number of 

 others of even more sweeping morphological importance. Stems have 

 been reduced and branching restricted : leaves are retained by some ; in 

 others, such as the prickly pears, they appear only as rudiments drop- 

 ping off before maturity, while in others, such as the great melon cacti 

 and the sahuaro, they are not visibly represented at all. So far does the 

 general reduction go in the Echinocacti or melon cacti that the adult 

 plant consists of a short stem, a few inches, or at most less than two 

 yards high, unbranched, and bearing only two types of spines which 

 may be taken to represent the rudiments of atrophied organs, or special- 

 ized organs, largely according to the morphological prejudices of the 

 observer. These plants represent the climax of specialization to desert 

 conditions and the end result of the influence of aridity on the develop- 

 ment of land vegetation. 



In these succulents which constitute the highest group of desert 

 plants, the cortex and medulla of the stems are exaggerated to an 

 enormous extent and the greater bulk of the plant consists of a paren- 

 chymatous tissue with mucilaginous cell-contents, which gives to the 



Fig. 2. A Group of Echinocacti and Ibervillea, isolated for Determination of 

 the Rate of Water Loss. Succulents of this type have osmotic pressures of less 

 than 12 atmospheres and absorb water only from soils containing large proportions of 

 moisture. Some of these individuals have been without an external supply for thirty- 

 eight months. 



