PROBLEMS OF THE DESERT. 35 



tion, etc., etc., are mechanical, inadequate and often falla- 

 cious. 



No simple illustration of the distributional movements, 

 or the travels of a plant may be brought to bear, but if we 

 were to place a few dozen drops of water, tinged with vari- 

 ous shades of color to represent a score of species on a large 

 board held level, some portions of which are polished and 

 some left rough, then imitate changes such as elevation and 

 depression, advance and retreat of the ice by tilting irregu- 

 larly in several directions, the drops would be seen to run 

 about, sink, merge, and vary in their grouping, sometimes 

 leaving a path of small globules behind, now joining to form 

 wet extended bodies or drop off the margin in a manner 

 highly suggestive of the actual fate of living organisms. 

 Some movements will be re-traceable to a limited extent and 

 some will not. 



Practically every investigation of the vegetation of the 

 desert touches upon the questions of water relations at some 

 point. Firstly, the comparatively uniform vertical distribu- 

 tion of moisture in the soil is accompanied by a fairly super- 

 ficial development of root-systems, the penetration for anchor- 

 age being very slight In conjunction with the absence of tall 

 forms. Not only are the roots of desert plants of great 

 lateral development, but they show a stratification in habit: 

 the roots of contiguous individuals occupying certain layers 

 within 4 inches of the surface, others of the next 6 inches, 

 while few go beyond the next foot in any density of branch- 

 ing. The conducting systems of these forms are often taxed 

 to the utmost, while the real conception of any practical use 

 in dealing with the physiology of desert plants is that of the 

 relation between the possible rate of evaporation from a 

 water surface and the actual rate from a leaf, which is known 

 as relative transpiration. 



A large number of species representing a score of the 

 great natural orders of plants exhibit a strongly developed 

 capacity for the storage and retention of surplus water in 



