iqiq] waterman— root SYSTEMS 31 



mature, but even then the evidence seems somewhat contradictory." 

 Benecke concludes that, on the whole, scarcity of chemical nutrients 

 tended to increase root length, calling this effect "hunger etiolation." 

 HovELER reached similar conclusions by growing plants in alternat- 

 ing layers of sand and humus. Seelhorst (35), by counting the 

 number of roots found in fertilized and unfertilized patches, decided 

 that ''in the fields investigated, plants strongly fertilized not only 

 produced stronger roots, but also roots penetrating to lower levels." 

 RuscHE (32) used various salts on 8 groups of plants and concluded 

 that the different groups responded somewhat differently, but that 

 on the whole sulphates produced the longest and nitrates the 

 shortest root systems. Tottingham made a thorough study of the 

 effects of various salts on young wheat plants in water cultures by 

 varying the proportions of the components of Knop's solution. He 

 included observations on length and weight of roots developed, 

 but drew no general conclusions. Dachnowski (13, 14, 15) and 

 RiGG (29) conclude that toxins in bog waters and in decaying rhi- 

 zomes respectively cause stunting of roots and therefore xerophily 

 or death of plants. 



ScHREiNER and Reed (33) conclude that roots of healthy grow- 

 ing plants excrete substances deleterious to root growth, especially 

 in plants of the same species. 



Conclusions from literature 



1. There has been comparatively little work done on extension 

 of root systems as such, and the value of the results is lessened by 

 the lack of uniformity in recording, which makes it practically 

 impossible to compare the results of different workers. The use of 

 vague descriptive terms in characterizing the branching of roots 

 is also unsatisfactory. 



2. Much variabihty of roots as a result of the action of the 

 environment is reported, but most of this action is destructive, as 

 the results of frost, drought, hard soil layers, etc. The experi- 

 mentation with chemicals shows differences in length and weight of 

 roots, but does not offer any definite evidence as to the causes 

 of root extension in general or of differential extension within a 

 single root system. 



