54 MINNESOTA BOTANICAL STUDIES, 



of the finer soil^ and the water is more easily available to the plant. 

 Clements, in "Research Methods in Ecology," (p. 78) says that 

 there is a pull exerted upon each water film by the soil particle itself. 

 This pull apparently increases in strength as the film grows thin- 

 ner, and explains why it finally becomes impossible for the root- 

 hairs to draw moisture from the soil. 



This property, like capillarity, is most pronounced in fine-grained 

 soils, such as clays, and is least evident in the coarser sands and 

 gravels. It seems to furnish the direct explanation of non-available 

 water, and in consequence, to indicate that the physiological water 

 cf the soil is an immediate result of soil texture. 



Soil Types. 



Three soil types and their combinations, together with the pure 

 types modified with compost, were used in this experiment. Series I 

 consisted of (a) loam soil, (b) loam with one-fourth compost 

 (bulk), (c) loam and one-fourth sand. Series II consisted of (a) 

 clay soil, (b) clay with one-fourth compost, and (c) clay with one- 

 fourth sand and Series III of a similar group of sand soils. Fig. 

 9 shows the typical growth of the plants in each series. 



The following table represents an average of all values obtained : 



There appears to be a slight tendency for the transpiration per 

 square centimeter to be depressed in the better soils. Series I 

 (b), II (b) and III (b) are pure types with one-fourth compost 

 added. Judging from the amount of green weight produced by 

 them, they may be considered best media for the growth of wheat 

 plants. The amount of transpiration per unit area is comparatively 



' Hedgcock, G. C. The Relation of the Water Content of the Soil to Certain 

 Plants, Principally Meeophytee. Rep. Bot. Surrey Neb., 6 : 48. 1902. 



