BIOLOGICAL PAPERS I4I 



weekly intervals between the collections as abscissae and the per- 

 centage of water present in the soil as ordinates (Fig. 2), it will 

 be seen that tlie moisture present in the soil is at all times more 

 than double the wilting coetficient. The interval between the 

 curve representing the soil moisture and the line representing the 

 wilting coefficient represents the amount of water available for 

 growth, and has been termed grozi'th zcater.* It will be apparent 

 that there is always a supply of growth water, although it must 

 be admitted that the amount available in July could permit no 

 ver}- heavy draughts, still the soil conditions must be regarded 

 as by no means very arid, — so mesophytic, indeed, do they appear, 

 viewed from the point of water supply, that one would be quite 

 at a loss to account for the very sparse vegetation upon any such 

 basis. A cause for the xerophytism may be found in the insta- 

 bility of the substratum, a factor fully discussed elsewhere by 

 Cowles and others. 



VEGETATIVE PRODUCTIOX. 



It has been previously stated that the cottonwood dune associa- 

 tion is one almost entirely dependent upon vegetative reproduction 

 for its maintenance. As the sand advances over the trunk and 

 branches of these trees, adventitious roots are given off, and 

 what was originally one tree becomes a group of several (Fig. 4), 

 each with its own root system. Doubtless the constancy of the 

 soil moisture supply is closely related to this condition of vegeta- 

 tive reproduction. A vegetation dependent upon such a method 

 of reproduction will, however, increase in amount very slowly, 

 especially as the ver}' instabilit}- which multiplies the trees by 

 burying them afterwards destroys them by removing the sand, 

 exposing the roots, and finally leading to their overthrow 

 (Fig. 3). In this uncovering process there is also a limited 

 amount of vegetative reproduction from adventitious buds which 

 arise upon the exposed roots at a distance from the parent plant 

 (Fig. 3). When the amount of erosion by the wind is limited 

 and soon checked, such adventitious shoots may result in the 

 production of a considerable number of new trees grouped about 

 the parent, but such reproduction is again to be regarded as lim- 

 ited in extent and of minor importance. These two methods, 

 however, do account for the permanency of the tree upon the 

 unstable substratum of the moving dune complex, although they 



♦ Fuller, G. D. Soil moisture in the cottonwood dune association of Lake Michi- 

 gan. Bot Gaz., 53: 512-514, 1918. 



