THE INTAKE AND UTILIZATION OF WATER 23 



water. Under these conditions some plants have survived submergence 

 for three weeks. The roots hved in this aerated water but grew more 

 slowly and in some cases root hairs developed. Even the roots of the 

 cocoanut, though often found in a practically saturated soil, are sensitive 

 to a lack of aeration, for thej^ thrive only if the water bathing them is 

 continually moving and they die if it is stagnant.^ It is not the water 

 but the lack of air in standing water that is harmful. The submergence 

 to wliich the roots of orchard trees are occasionally subject in certain 

 locations is of a type that is generally accompanied by a lack of aeration. 

 The result is a prompt killing of the root hairs, followed more or less 

 closely by the death of the roots themselves. This is likely to be the 

 case when roots are submerged by the rise of the ground water in irrigated 

 sections and in orchards planted in low-lying poorly drained ground. It 

 is not so apt to be the case with trees planted on low but well drained 

 bottom lands, or alluvial soils subject to occasional overflow of short 

 duration during periods of flood. Even in the latter instance, however, 

 it is noteworthy that the trees are apt to be severely injured, or killed, 

 if the roots are submerged for more than several days during the growing 

 season or for a period of as many weeks during their dormant season. 

 Certain bog plants like the swamp blueberry and cranberry, however, 

 will stand complete submergence of their root systems for a period of four 

 of five months during the winter, though submergence of as many days 

 during the growing season is attended with great risk.^'' There is good 

 reason to believe that many of the troubles variously attributed to 

 winter injury, drought and soil infertility may be end products of tempo- 

 rary root submergence that leads immediately to a kind of root pruning. 

 It should be realized that root systems may be submerged though no 

 water stands on the surface of the soil. More attention is devoted to this 

 phase of the subject in a later chapter of this section and in the section 

 on Temperature Relations. 



TRANSPIRATION 



Large quantities of water are lost by evaporation from. the portions 

 of the plant above ground and particularly'- from the leaves. Duggar*^ 

 estimates that an apple tree 30 years old might lose 250 pounds of water 

 a day or possibly 36,000 pounds a season. At this rate an acre of 40 

 trees would represent a water elimination of 600 tons a year. This 

 water loss from plants is not strictly a physical process of evaporation, 

 because it is influenced by factors such as light and is subject to some 

 degree of modification by the plant. Since evaporation is not affected 

 in the same way by these factors the water loss of plants must be con- 

 sidered a physiological process; hence it is designated transpiration. 

 Whether transpiration performs any useful role in aiding the process 

 of absorption of mineral constituents, or in lowering the temperature 



