iqiq] waterman— root systems 49 



into the zone of influence of these plant parts seems to cause elonga- 

 tion of the roots of some species, but inhibition in those of others. 

 The determining factor here might be a direct chemical stimulation, 

 the furnishing of organic material, or in opposite cases the presenta- 

 tion of some injurious or inhibiting chemical product of decay. 

 Xo deciding evidence has as yet been secured along this line. There 

 were some indications that the roots of seedlings are more sensitive 

 to inhibiting factors than are those of mature plants. 



Oxygen. — There was little evidence of unequal distribution of 

 oxygen through the dune substratum. The only exception would 

 be in connection with a high water table, and there the evidence 

 was not conclusive. The slight effects observed were inhibitory in 

 nature, and, in one case at least, referable to destructive action of 

 micro-organisms rather than to a direct reaction of the plant tissue 

 to the absence of oxygen. 



Penetrability of the soil. — It is quite possible that the substratum 

 may be more penetrable in some localities, either on account of 

 differing densities of certain layers or of the disintegration of buried 

 plant parts. This would be difhcult to prove either way, but there 

 was no evidence of any tendency of the root tips to be turned back 

 by a less penetrable layer when accidentally wandering too near 

 to the borders of a dark layer, as would be the case if the dark layer 

 had been the more penetrable medium. In fact the only observed 

 case of a probable difference in penetrability was that of a soil 

 layer which apparently contained a percentage of wind-blown clay. 

 In this case certain roots were distributed along the upper surface 

 of this layer when normally they might have been expected to pass 

 directly down through it. As already indicated, the difference of 

 penetrability of the moist sand under the soft sand mulch may be 

 the determining factor in the distribution of Artemisia laterals. 

 This distribution, however, may also be explained by the difference 

 in moisture content of the two layers of sand. 



There is no clear indication as to the method by which the chemi- 

 cal substances act on the root, whether by direct stimulation or by 

 removal of some inhibition; neither are the relative roles of organic 

 and inorganic substances more than suggested. The cases of 

 marked elongation of roots in one direction would seem to indicate 

 the possibility that roots under certain conditions can make use of 



