160 WATER 



There are some plants that change from one group to another at 

 different seasons. Familiar examples of this phenomenon are the 

 deciduous trees of the temperate zones. The majority of these 

 trees, such, for example, as Acer saccharum, the sugar maple, are 

 typical mesophytes in summer when they have leaves but are 

 xerophytes in winter when they are leafless. Such plants are called 

 tropophytes. 



Some of the characteristic differences between xerophytes, meso- 

 phytes, and hydrophytes have been brought out in the discussion 

 of the ecology of stems and leaves and others will be taken up in a 

 later chapter. It is necessary to point out here, however, that this 

 classification of plants does not depend so much upon the actual 

 amount of water present in the immediate environment as upon the 

 relation between absorption and transpiration. In some cases the 

 cause of xerism is an actual inadequate water supply, as in deserts, 

 but in other cases it may be due to physiological dryness; that is, 

 to difficulty of absorption, as in saline regions or in peat bogs; or 

 it may be due more largely to high transpiration as in some alpine 

 regions. The chief causes of hydrism, on the other hand, are a 

 saturated atmosphere or an abundance of soil water, or both. 



103. Transpiration.— When a land plant is in an active condition 

 there is a more or less constant stream of water passing through it. 

 The water is absorbed into the roots, transported upward through the 

 xylem, and transpired from the surfaces of the leaves. The plant 

 actually uses a considerable amount of water but it absorbs and 

 transpires a great deal more than it needs for its ordinary metabolic 

 processes. The amount passing through the plant varies greatly at 

 different times and sometimes is surprisingly large. It has been esti- 

 mated for example that a beech tree, one hundred years old, trans- 

 pires about 60 barrels of water during a season, and that a field of corn 

 transpires during its entire season of growth as much water as would 

 amount to 5 inches of rain on the field. Actual measurements during 

 a period of ten days of dry weather have shown that corn transpired 

 from 6 to 9 times its own dry weight daily while alfalfa transpired 

 from 36 to 56 times its own dry weight. On the basis of 1 ton of dry 

 matter per acre this would amount to a daily loss of 0.05 to 0.08 of an 

 acre inch of water daily from the cornfield and of 0.32 to 0.49 of an 

 acre inch from the alfalfa field. 



Of course all plants do not transpire so rapidly as those we have 

 cited. It has been estimated that sun mesophytes, such as alfalfa, 

 often transpire at least 175,000 times as rapidly, per unit area, as 



