BOOKS AND CURRENT LITERATURE 



Root Habits of Desert Plants. In a recent publication 1 by 

 Dr. W. A. Cannon we have a highly interesting as well as exhaustive 

 consideration of the root habits of a variety of desert plants. After 

 a preliminary treatment of the moisture and temperature conditions 

 of the soil in the region around Tucson, Arizona, the author enters into 

 a discussion and description of the root systems of upwards of sixty 

 species which are characteristic of that locality. They are considered 

 under three heads as follows: summer annuals, winter annuals and 

 perennials. Among the annuals there is a striking difference between 

 those which flourish during the summer rains and those whose growing 

 season is during the time of winter precipitation, although both have 

 about the same depth of penetration of the roots. Characteristic of 

 the latter are the prominently developed tap roots with only a relatively 

 small development of laterals, while the former have a much more gen- 

 eralized type of root system. In the summer annuals the laterals are 

 well developed and the main root itself is frequently forked, affording 

 thereby a greater absorbing surface than is found in the winter forms. 



As a working hypothesis to explain these conditions the following is 

 suggested. In the summer the conditions, both sub-aerial and sub- 

 terranean are suitable for rapid growth and, the most favorable condi- 

 tions for water absorption being offered, there is the resultant develop- 

 ment of the root system. In the winter, on the other hand, the soil, 

 to an appreciable depth, is cooler than the air in the day time, thus 

 retarding root development. As a result the growth of the shoot is 

 limited, and this in turn demands less of the roots. In the opinion of 

 the author it seems to be the relative difference between air and soil 

 temperatures in winter and in summer which is to be regarded as the 

 critical factor. In summing up the question he states that it may be 

 said that the summer type of root system tends towards hygrophily, 

 while that of the winter annual has a rather xerophilous inclination. 

 Such of the winter annuals as have a more generalized root system, and 

 there are a few, are limited to regions where the soil conditions are 

 especially favorable. 



1 Cannon, W. A., Root Habits of Desert Plants. Carn. Inst. Wash., Publ. 

 131, 1911. 



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