BIOLOGY OF CACTI 



45 



it does not grow at all, or rarely, on the mesa where the rock-like 

 calliche forms a thick and nearly impenetrable stratum which 

 reaches almost to the surface. However, it may not he wholly 

 a problem of anchorage, since the morphological condition may 

 be associated with a physiological one, as for instance, subirriga- 

 tion or proper drainage which may be indispensable factors in 

 its water relations. Although the character of the root system 

 may thus be closely connected with the character of the habitat, 

 certain features in the local distribution indicate that it cannot 

 be too narrowly insisted upon. For example, Cerem giganteus 

 avoids northern slopes, although to all outward appearances the 

 structure and the water supply may be quite the same as on the 

 other sides. 



Echinocactus presents quite a different condition of affairs. 

 The plant does not require unusual protection against lateral 

 stresses. It grows most abundantly in this locality on the mesa 

 where the soil is shallow. The roots are so placed that they can 

 neither afford safe anchorage for a tall plant, nor absorb water 

 at the water level. There is therefore a direct relation between 

 the character of the plant and that of the root system, on the 

 one hand, and the character of the root system and that of the 

 habitat, on the other. It should also be noted that the roots 

 of Echinocactus, which are very shallowly placed, permit the 

 plant to derive benefit from relatively small rains, but, by the 

 same token, that they prevent it from getting water other than 

 what falls on the area included by them. 



2. The striking disproportion between absorption and tran- 

 spiration, which w^as observed in Opuntia versicolor, is thought 

 to be of great importance in accounting for the distribution of 

 the plant (and perhaps of the family) in those parts where evap- 

 oration greatly exceeds preci})itati()ii. 



versicolor and Evhiuoravtus iriMlzoil (luriii-r periods of prolonged 

 drought. At the time of the siiininer rains the rate was greatly 

 increased and in all instances the increase was a^xiciated with 

 the renewal of growth. 



4. A direct relation was observed between structure and tran- 

 spiration. The mature portions of Echinocactus and of Opuntia 



