iqiq] waterman— root systems 47 



that environment, and that when germinating in soils richer in 

 nutrients these root systems would be less extended. On the other 

 hand, species which had similarly become adjusted to the richer 

 soils had thereby become dependent on these soils for the 

 development of extended root systems, and were therefore stunted 

 in sandy soils. While a relation to past conditions may not be the 

 true explanation for this specificity, the facts are evident and cer- 

 tainly indicate very definite relations to present conditions. These 

 should be of great value in selecting species for revegetating exposed 

 areas and other localities where the humous content of the soil is 

 slight or unevenly distributed, as well as in the cultural treatment of 

 species which may be regarded as worthy of development for their 

 economic value. 



On this basis it would follow that general statements in regard 

 to the root habits of dune plants as a class are dangerous, and that 

 the so-called ''dune pioneers" are not all on the same footing; 

 that in fact they should be put into two widely different groups with 

 a series of types occupying intermediate positions. In one group 

 would be the Prunus pumila type, which does not have the power 

 of extending root systems widely in pure dune sand, but is stunted 

 and does not reach maturity unless its roots find buried organic 

 matter. In the opposite group would be the Ammophila type, 

 which reaches maturity in pure dune sand and whose root system is 

 limited in extension by the presence of decaying plant materials. 

 Similar to Ammophila would be Artemisia, Cirsium, and Campanula, 

 with Calamovilfa and other grasses, and probably species of Solidago 

 and Aster. Lithe spermum is similar ecologically, although with a 

 very different root habit, while Lathyrus maritima occupies a 

 peculiar position on account of its relationship to the nodular 

 bacteria. From these observations it would seem that Ammophila 

 is the only plant which can reasonably be expected to thrive suc- 

 cessfully on the normal dune substratum. 



In seeking for the causes of the asymmetric development of root 

 systems observed, it is evident that the only factors to be considered 

 are those working in the soil and exerting an unequal or one-sided 

 influence in the system as a whole. These may be limited to four, 

 namely, moisture, chemicals, oxygen, and density or penetrability 



