28o BOTANICAL GAZETTE [october 



i . 25 per cent or more. This greater constancy is attributed to 

 the moving mulch of dry sand which breaks the continuity of 

 surface films in the soil particles, thus preventing rapid evapora- 

 tion. This ability of a plant to maintain a higher transpiration 

 index with a growth water content of 1.25 per cent than the same 

 species does with 20 per cent or higher, such as is commonly true 

 for clays, indicates that the amount of growth water has but little 

 relation to the transpiration index so long as the soil moisture 

 content remains above the wilting coefficient. This gives a some- 

 what unusual aspect to the question of mesophytism and the 

 part played by soil water as a factor in plant growth. So far 

 as Tilia is concerned, it produces more vigorous vegetative struc- 

 tures, which retain their activities later in the summer, when 

 growing in open situations on sand than when in the forested 

 dune complex; and the factor of greatest importance seems to 

 be, not the average growth water of the soil, but whether the 

 available moisture repeatedly falls below the wilting coefficient. 

 So long as it is above that point, although it is by only a very small 

 percentage, the normal activities, including high transpiration, 

 are carried on. 



There is one point concerning this relation which needs investi- 

 gation, however, namely, the relative extensiveness of the root 

 systems in the two situations. I am inclined to the idea that the 

 ability of Tilia to develop adventitious roots when covered by 

 an advancing dune may enable such individuals to draw their 

 water supply through a root system the absorptive surface of 

 which is greater in proportion to the amount of foliage than is 

 true of this species when growing in the forest complex. Another 

 point of probable difference is that the individuals on the open 

 -and may obtain a considerable portion of their water from a 

 greater depth than do those on the humus; but the extent to 

 which the root systems persist when submerged by advancing 

 dunes has never been worked out. 



As indicated by the averages shown in fig. 1 1 , the situation at 

 E is distinctly more xerophytic than at A. Every fact recorded 

 in the experimentation points to this conclusion. The size, 



