50 BOTANICAL GAZETTE [july 



organic matter directly; in other words, that green plants may be 

 somewhat saprophytic in nature. A careful microchemical study 

 of the contents of the root at different points along its length would 

 be necessary in order to get any definite information on this point. 

 It would probably occur only when the root found materials exactly 

 suited to its needs, and would be proportionately greater as the 

 increasing length of the root made transportation to the shoot 

 increasingly difficult. 



Under these stimulating influences abnormal lengthening and 

 thickening of roots occur to such an extent as to call in question the 

 value of the common method of estimating root development by 

 measuring the length and weight of roots. In fact, if the explana- 

 tion of direct local use of organic material of the root be accepted, 

 the abnormally long root may be a detriment instead of a benefit 

 to the plant as a whole. It would seem as though there could be 

 developed some method of evaluating the absorbing power of roots 

 through study of the structure of the different parts of the root 

 system which would give more dependable results than the length 

 and weight method. 



The evidence cited emphasizes the unique character of the 

 dune substratum, in that pure dune sand is the only soil in which 

 mineral salts, with the exception of calcium carbonate, are prac- 

 tically absent, and organic matter is so rare and scattered that as a 

 general factor it is practically negligible. This is in strong con- 

 trast with many arid deserts in which large quantities of desirable 

 mineral salts are present, needing only the addition of water to 

 make them available for plant use. While irrigation is the main 

 need of many desert stretches, it would not solve the problem of 

 plant culture on a dune area. 



Summary 



I. The study of the literature shows that the work done on 

 extension and development of root systems has been surprisingly 

 little, in view of the importance of the root in the utilization of 

 moisture and chemicals in the soil. This study also emphasizes 

 the necessity of interpreting the extension of root systems in the 

 light not only of structure and functions, but also of the causes of 

 such extension. 



