WATER UTILIZATION BY TREES 55 



woolly and silky leaves increases on the southern slopes and in propor- 

 tion to their dryness. 



In the section dealing with the effects of temperature and moisture 

 on water content, the correlative changes in osmotic pressure of the 

 cell sap were noted. Cells with high osmotic concentration are not 

 so permeable to water, and plants in dry localities commonly have a 

 high concentration of sap in the leaf cells. Which is cause and which 

 is effect in this case is not so easy to determine. Do the plants have 

 a high concentration of cell sap in order to check transpiration, or has 

 the excessive transpiration resulted in an increase of the osmotic 

 pressure of cells? The two effects are probably simultaneous and re- 

 ciprocal. This is an excellent illustration of the fact that an unfavor- 

 able environment may cause a reaction in the plant which checks the 

 unfavorable effect. 



Other substances in the cell, in addition to salts, may check tran- 

 spiration. Among these may be latex, since, as Volkens (222) points 

 out, the number of plants with milky juice is relatively high in hot 

 and dry climates. It might also be mentioned that, among the maples, 

 the only one with a milky sap is the Norway maple, which grows in soil 

 physiologically drier than the other maples. 



To tannins also has been attributed the function of checking tran- 

 spiration. The leaves of evergreen plants must lower their transpira- 

 tion as much as possible, and Fleischer (59) considers that the tannins 

 which are present in almost all evergreen leaves, especially in the 

 epidermis, are a protection against drying out in winter. Volkens 

 (222) points out that tannins are also common in desert plants. 



The value of tannins in checking transpiration has been questioned 

 by Wisser (243), who pressed the sap from the leaves of plants rich in 

 tannins and compared the evaporation rate of this sap with that of 

 distilled water; he found that the cell sap evaporated 93 to 99 percent 

 as rapidly as the distilled water, which would seem to indicate that 

 the tannins are not such a good protection against transpiration as 

 others have thought. It should be pointed out, however, that Wisser's 

 test is not a particularly good one. When whole cells are squeezed, 

 one gets not only the" water from the vacuole but also from the proto- 

 plasm, and the concentration of the tannin would consequently be less 

 in the pressed product than in the vacuoles of the living cells. 



An especial protection against strong evaporation is the formation 

 of buds inside the base of the petioles, as in Philadelphus and Platanus, 

 to which Wiesner (240) first called attention. According to him, also, 

 the terminal buds of many species of Acer are protected in a similar 

 way against drying out, since here the bud remains protected for a long 

 time by the base of the two top leaves. Also, the stipules commonly 

 protect the young, tender leaves of the bud from excessive water losses. 

 Interesting in this connection is Liriodendron tuliipijera L.; here the 

 two relatively large, shield-shaped stipules are so placed that they 

 form a kind of sack, in which the foliage leaf grows protected as in a 

 sort of chamber until the epidermal cells have become thickened 

 enough so that there is no longer any danger of drying out, when the 

 two stipules separate and fall off. 



Plants growing in dry habitats show fewer stomata than those 

 growing in moister regions. Czech (41) found that species growing 

 in wet sites had more stomata per square millimeter than related 



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