48 BOTANICAL GAZETTE [july 



of the soil. In the case of a few plant groups, as the willows, which 

 are recognized as having a hereditary hydrotropic tendency, there 

 will be little question as to the dominant factor, but the evidence 

 in the cases of the species previously described must be considered 

 somewhat carefully. 



Taking up first the possibility of moisture as the main factor, 

 it should be noted that below the first few centimeters water is 

 evenly distributed in dune sand, and cannot be regarded as the 

 causal factor in the development of the asymmetric root systems 

 described. It might be assumed that the following of humous layers 

 and dark patches by plant roots should be attributed to the presence 

 of a greater amount of moisture in those patches than in the adjoin- 

 ing sand. Granting the power of humus to absorb greater amounts 

 of water than can the pure sand, it would be difficult to prove that 

 water was held by these patches beyond the quantity required by 

 their increased wilting coefficients. The patches are so small and 

 the medium so unstable that it would be practically impossible to 

 collect from adjoining patches pure samples large enough for 

 accurate determination of their respective moisture contents and 

 wilting coefficients. In the experimental work the adjustment of 

 the water supply was a difficult matter. As a result there were 

 occasional reactions which seemed to point to the influence of 

 moisture, but even in these it could not positively be stated that 

 chemical influence was not the dominant factor with moisture as 

 contributory only. 



Approaching the question from the chemical side, we find 

 definite evidence, both from observation and from experimentation, 

 that, with the species considered, variations in chemical solutions 

 produced changes in root development, while variations in water 

 supply produced little or no evidence of such changes in develop- 

 ment. Perhaps the most conclusive evidence was found in the case 

 of a patch of humus in a pot culture of Prunus pumila. Here 

 the abnormal development of laterals occurred under the patch of 

 humus in a zone stained by leaching of organic matter rather than 

 in the free region on either side, where the moisture content should 

 be the same as in -the region under the organic patch. 



We find also marked differences in elongation of root systems in 

 the presence of decaying plant parts. Contact with or even coming 



