44 



BOTANICAL GAZETTE 



[JULY 



In several cases there were indications that taproots and 

 laterals gave different reactions to the same stimuli. In these 

 species taproots were never deflected by horizontal black layers or 

 plant parts, while laterals were apparently free to move in any 

 direction. Typical species showing this habit were Artemisia and 

 Cirsium Pitcheri, while in Prunus pumila and notably in Campanula 

 the taproots, if such they could be called, showed marked variabil- 

 ity in a horizontal direction. Whether this difference was structural 



--\; / (•■ 



Fig. is 



Fig. 1 6 



Figs. 15, 16. — Fig. 15, P. pumila root system after 2 months' irrigation with 

 Knop's solution; round black spot represents location of center of diffusion and 

 dotted circle apparent limits of influence; practically all new roots found within the 

 circle; fig. 16, Thuja occidentalis: new root tips grown under irrigation with Knop's 

 solution. 



or due to a difference in quantity or quality of the stimulus was not 

 evident. ^ 



EXPERIMENTATION 



A consideration of these observations would suggest several 

 possible factors as causes for the observed characteristics of root 

 systems of dune plants. Chief among these would be the distri- 

 bution of chemical materials not normally found in dune sand, also 

 the distribution in the sand of moisture and of oxygen and the 

 varying penetrability of the sand. As the chemical factor seemed 



