76 THE WATER-BALANCE OP SUCCULENT PLANTS. 



The habit of accumulation of a large water-balance affects some forms 

 in which the reduction of the shoot has been carried to its gfreatest extent, 

 and the forms displaying both modifications, such as the massive euphor- 

 bias and cacti, constitute the most pronounced types of desert vegetation. 

 It is to be seen, however, that succulency is manifested by many plants, 

 in which the primitive xerophytic modifications have not been extensive, 

 and a water-balance is carried in roots, stems, leaves, and special org-ans. 

 The inference seems a fair one, therefore, that succulency is not the 

 result of the simple causes leading to xerophytism. 



A review of the conditions connected with the existence and distribu- 

 tion of succulents shows that they are abundant in northern and southern 

 Africa, certain deserts in .South America, and especially numerous in the 

 arid regions of western and southern North America, reaching a max- 

 imum development as to size and number of species in the elevated basins 

 and bolsons of southern Mexico, where several species of great tree-cacti 

 are prominent in the landscape. The regions characterized by succulents 

 have a soil often rich in lime, in which the precipitation is received in 

 regularly recurring seasons. Cactaceae and Crassulaceae may thus be 

 found down to the limit of the spring tides in the North Temperate Zone, 

 while a few of these and other succulents occur far north in rain-forests 

 and very cold regions. An analysis of the conditions mentioned is not to 

 be allowed to account for succulency, however, since one group of plants 

 showing the capacity for accumulation of water, the halophytes, inhabit 

 saline shores and soils around the world and through a wide range of lati- 

 tude. The only invariable conditions attendant upon the development of 

 succulents seem to be the existence of an abundant supply of moisture in 

 the soil or substratum during certain seasons and the presence of solu- 

 tions of high osmotic activity in contact with the absorbent organs . Some 

 halophytes occur in localities in which the soil-solutions are continuously of 

 high concentration, while in other cases they may be subject to a wide 

 range of variation by floods and tides. 



An examination of the chemistry of these forms might probably lead to 

 results of value in the interpretation of their development. The cacti of 

 the Tucson region, and probably all of these forms, are rich in calcium 

 carried in solution in the sap as an accident of its occurrence in abundance 

 in the soil. The sap shows a high degree of osmotic activity, ranging 

 from 5 to 12 atmospheres in various species in a state of maximum tur- 

 gidity, to perhaps twice this pressure when the water-balance is depleted. 

 Furthermore, these plants, especially the halophytes, are known to be 

 capable of an accommodative reaction by which the osmotic pressure may 

 be automatically increased in response to the increased concentration of 

 the soil-solutions. 



The sap of succulents is characterized by a high degree of acidity, 

 which seems to be least in Echinocadus and greatest in Opuntia versicolor 



