191«J FIELD CROPS. 825 



Varieties of corn grown continuously for many years under humid and under 

 relatively dry conditions were found to have approximately the same average 

 water requirement per pound of dry matter. The extreme variation in the water 

 requirement of 11 varieties of corn differing in acclimatization and habits of 

 growth was 230 lbs. and 296 lbs. per pound of dry matter, although most varieties 

 were rather uniform in this regard. Several varieties with alleged special 

 drought resistance possessed practically the same water requirements per 

 pound of dry matter as the average of all the 11 varieties tested. The water 

 requirement for milo maize was the same as the average for the 11 varieties, 

 but for Black Amber sorghum it was considerably higher. Wild sunflowers ex- 

 hibited a water requirement per pound of dry matter approximately double 

 that of corn and a total water use somewhat greater than that of three corn 

 plants. 



In a study of the intake of soil solutes data were obtained concerning the 

 relation between transpiration and ash content as affected by atmospheric 

 humidity, seasonal climatic differences, soil fertility, soil moisture, kind of crop 

 and variety, and limitation of the amount of soil through the size of the potom- 

 eter. An increase in the greenhouse in atmospheric humidity, lowering the free 

 water evaporation 47 per cent, reduced the amount of water transpired per gram 

 of ash content 38 per cent and per gram of dry matter 40.5 per cent. A natural 

 climatic difference, lowering the free water evaporation 40 per cent during July 

 and August in 1913 as compared with 1914, reduced the transpiration per gram 

 of ash content 22 per cent and per gram of dry matter 27 per cent. Under other- 

 wise equal conditions the amount of water used per unit of dry matter fell, and 

 the amount of solute taken in per unit of water transpired, as well as the 

 amount taken into the plant, the dry matter produced, and the total amount of 

 water transpired, rose with the availability of the soil solutes in different 

 degrees of soil fertility. As compared with an abundance of moisture, less water 

 was transpired per gram of ash content and also per gram of dry matter in a 

 relatively low soil saturation. The influence of the size of the potometer was 

 brought out by the fact that in potometers containing 32.5, 85, 150, 239, 583, and 

 956 lbs. of soil the amount of transpiration per gram of ash content was 6.14, 5.7, 

 5.2, 5.5, 5.07, and 4.32 kg., respectively. From a review of all the data the 

 author believes it may be concluded that at least within the practical limits of 

 crop production, other things being equal, an increase in the density of the soil 

 solution is accompanied by an increase in the amount of solute taken into the 

 plant per unit of water transpired. 



Considerable variation was determined between the different varieties of corn 

 and sorghum with reference to the thickness of the leaf and of the epidermis, 

 and also in the number of stomata per unit of leaf area, but without an apparent 

 striking or consistent correlation with the transpiration rate per unit of dry 

 matter or per unit of leaf area of the different varieties, and also without strik- 

 ing response in the relative number of stomata to variation in either soil 

 moisture or soil fertility. As an average for 11 varieties of corn, a plant having 

 949 sq. in. of leaf area had 104,057,850 leaf stomata, which occupied in the 

 epidermis of both sides of the leaf, when open, 1.52 per cent of the area of the 

 leaves. The entire epidermis comprised 30.8 per cent of the leaf thickness. 



" Transpiration appears to be a purely physical phenomenon, depending prim- 

 arily upon the moisture supply in the leaf and the evaporating power of the 

 atmosphere, which is modified in some degree by temperature effects resultant 

 within the leaf from chemical activity, transpiration, and from the absorption of 

 radiant energy." 



[Report on the progress of farm crops investigations] {Missouri Sta. Bui. 

 141 (1916), pp. 29-31, 36-39, fig. 1). — In wheat breeding investigations con- 



