THE BOOT-SYSTEM OF DESERT PLANTS 99 



species of opimtia (Opuntia vivipera) in which the roots of one and the 

 same individual may be either fibrous or fleshy. Also species resembling 

 Op tint ia arbuscula, but possibly another species, which grows in the 

 vicinity of Sacaton, Arizona, appears to have fibrous roots only. It has 

 been found also that the seedlings of many cylindro-opuntias have 

 fleshy roots. This last may be taken to be a temporary or juvenile 

 stage, but probably is not, for reasons which will appear directly. With 

 the above and other observations in mind, specimens of opuntias of 

 several different species have been grown in saturated soils, with the 

 uniform result that the roots formed in the saturated soil were fleshy. 

 This result might be taken to indicate the immediate effect of an abun- 

 dant water supply, but in the end it may be found that the result, in 

 part at least, may be attributed to the air relation. 



There is another relation which has not been referred to and which 

 is of great importance, namely, the osmotic relation. This can be 

 given briefly. A strong impetus to the study of this relation has re- 

 cently been given by Fitting, 2 who has shown that the shoots of certain 

 desert plants may possess a very dense cell sap, so concentrated in fact 

 that an osmotic pressure as great as 100 atmospheres has been deter- 

 mined, which pressure may even be exceeded. In the cells of the shoots 

 of ordinary mesophytes the usual pressure is said to be from 5 to 11 

 atmospheres. While it has not been shown that the cell sap of the root 

 hairs of such desert plants as have high osmotic pressures in the cells 

 of the shoots is isosmotic with them, yet it has been assumed that the 

 roots of these plants contain a very dense sap, as is probably the case. 

 There is an apparently direct relation between the dryness of the 

 habitat and the concentration of the plant juices, by reason of which 

 the desert plant can absorb water from an intensely dry soil. As a rule 

 the highest osmotic pressures, therefore, are to be found among peren- 

 nials living in the driest situations, and during the most arid seasons. 

 From this condition it is of interest to note that it is probably those 

 plants in which the generalized type of root-system is to be found, or a 

 type approaching this, that possess the most highly concentrated cell 

 sap, since it is plants having this form of roots, as was noted above, 

 which occupy the most arid habitats. We may conclude from this 

 additional evidence that, so far as the Tucson desert is concerned, it is 

 not the most deeply penetrating type of roots which are to be considered 

 the desert form par excellence, but, quite the contrary, it is such a root 

 as can both reach out widely and penetrate as deeply as the soil permits 

 find in which there is developed a cell sap of extremely high concentra- 

 tion. 



2 ' ' Die "Wasserversorgung imd die osmotischen Druekverkaltniss der Wiisteu- 

 pflanzen," ZeitscJi. f. Bot., 4, 1911. 



