46 BOTANICAL GAZETTE [july 



Pot cultures with nutrient materials. — Pot cultures with 

 P. pumila seedlings were grown in dune sand watered (i) with dis- 

 tilled water, and (2) with Knop's solution, and (3) in a substratum of 

 rotted barnyard manure. The results are shown in fig. 17. While 

 the root systems in (i) and (2) seem rather similar in extension, an 

 examination showed that (2) was more closely branched than (i) 

 and had nearly twice as many absorbing root tips. The striking 

 feature of (3) was the much greater development of the shoot. 

 Attempting to reproduce more closely the natural conditions, a 

 seedling was grown in the laboratory in a pot of sterile dune sand in 

 which a small patch of rotted manure had been included. Here 

 the most marked extension of rootlets appeared under the patch of 

 organic matter in a space stained by leaching from the material 

 above. 



Pot cultures with inorganic salts. — Experiments on the 

 directive influence of inorganic salts have showed so far only indica- 

 tions and suggestions. Rusche (32) has already shown differences 

 in extension produced by different salts, and attempts were made 

 to extend his results along the line of directive influence. The 

 technical difficulties of finding suitable seedlings and of presenting 

 the chemicals in solution from one side only, along with the uncer- 

 tainty as to the direction and extent of dififusion of the solution 

 through the sand, have so far prevented reaching any conclusive 

 results. Indications of a tendency to react'in accordance with the 

 sequence of the liatropic series were observed and definite specific 

 differences were also evident. 



Discussion and conclusions 



The most marked feature of these observations is the varied 

 reactions of different species to apparently identical environmental 

 conditions. These showed sufficient constancy within a species, 

 but with such differences in different species as to suggest that the 

 reactions are specific and that the controlling factor is heredity. 

 Their general character also strongly suggests their relation to a 

 former habitat or condition of growth. It would seem possible 

 that species which had varied in the direction of an extended root 

 system in sandy soil had survived there and had become adjusted to 



