ROOT SYSTEMS 33 



The roots show a marked lateral development a few inches 

 below the surface, though there may be, in addition, a well- 

 branched system penetrating 2 or 3 ft. to the maximum 

 depth of the subsoil. The gravel surface layer preserves 

 an even moisture in the underlying soil. The " half-gravel 

 slide " community exists on stabilised slopes with a deeper 

 and richer soil. Here again the surface root system is well 

 developed, but is more markedly supplemented by a deep and 

 well-branched system draining water from depths of 3 ft. 

 A forest community with Pinus ponderosa and Pseiidotsuga 

 mucronata finally supplants the half-gravel slide community. 

 The soil here consists of i to i| ft. of rich humus covered 

 by litter, and the root systems of the undergrowth plants 

 are almost entirely confined to this shallow region. 



Roots of Desert Plants. — The root systems of desert 

 plants, exposed as these are to the most extreme arid condi- 

 tions, have always attracted attention. The most extensive 

 investigation is that carried out by Cannon (191 1) in the 

 neighbourhood of the Desert Laboratory at Tucson, Arizona. 

 In this region, with a rainfall of about 11 in. occurring 

 chiefly in midwinter and midsummer, extreme desert con- 

 ditions are not exhibited, but, except in the flood plain 

 of the Santa Cruz River, there is no water available for plant 

 growth in the upper layers of the soil within a few weeks 

 after rains. The summer rainfall is torrential, and there is a 

 large run-off", while in winter the rainfall is better distributed 

 and absorbed. There are over two hundred species of 

 annuals, of which one-fifth have their growth period in 

 summer, and the remainder in winter. None seem to 

 possess a deep root system, the depths measured being always 

 under i ft. The winter annuals have in general a well- 

 developed tap root with sparing laterals. The summer 

 annuals, on the other hand, belong to Cannon's " generalised " 

 type with vigorous and much-branched laterals. This 

 difference is related by Cannon to the more favourable 

 conditions of absorption in the warmer summer soil which, 

 along with temporarily high atmospheric humidity following 

 the torrential rains, promotes shoot development and 



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