The Root Habits of Desert Plants. 



INTRODUCTION. 



It is now generally recognized that the arid and semi-arid regions are 

 especially favorable areas in which to study the habits of plants. This is 

 partly because the vegetation of the less humid countries represents the 

 most advanced type of land forms — that is, those farthest removed from 

 the primitive water-loving plants — and partly because, as the environ- 

 mental features are severe, so the response on the part of the plants is corre- 

 spondingly effective in order to bring about survival. The shoot habits of 

 desert plants have received considerable attention from botanists, but the 

 reaction of roots to desert conditions has, in a large measure, been neglected. 

 That this neglect is illogical and without good reason is apparent when it 

 is recalled that the connection between the plant and a most important 

 feature of the environment, the soil and its water content, is sustained 

 only through the roots. 



The fact is well known that the roots of plants have a twofold function. 

 They at once afford safe anchorage and support and at the same time are 

 the means by which water and inorganic food materials are acquired. The 

 dual nature of roots does not find separate organic expression in the most 

 primitive plants, and in certain of them, the algae, organs answering to 

 roots serve the purpose of anchorage only. In the low land plants, the 

 roots, morphologically rhizoids, are, from this point of view, quite undif- 

 ferentiated, but as the scale of plant life is ascended we find these functions 

 separated both in time and in space, at first on the same root and its 

 immediate branches, and finally there is differentiation in the root-system 

 of the plant, by which the anchorage is largely taken over by one set of 

 roots and the absorption by quite another set. The last is the advanced 

 condition found in the extreme xerophytic desert plants, such as many of 

 the cacti. Thus the extent and the character of root development will 

 reveal, in a measure, the degree of xerophily of a plant. 



The prevailing idea that the roots of plants of the deserts, or of semi- 

 arid regions, are of great length, especially that they penetrate the ground 

 to great depths, doubtless has its origin largely in the belief that desert 

 plants are obliged to develop a deeply placed root-system in order to obtain 

 water during long dry seasons, and also in the few striking examples of 

 really long roots of plants of arid countries which are accented as repre- 

 senting the root condition of all desert plants. 



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