CLIMATIC FACTORS 127 



rich in humus acids. No characteristic xeromorphy is demonstrable 

 in deciduous Ericaceae of peat moors or in many other plants of high 

 moors. Indeed, comparative studies have shown that many 

 Sphagnum moor plants exhibit decided hygromorphy. 



Loss of water by plants is increased by guttation. Water stomata 

 (hydathodes) excrete liquid water in drops. Since, according to 

 Schimper, guttation is especially common in warm, moist, tropical 

 forests,^ it is easy to conclude that this is a partial substitute for the 

 reduced transpiration. According to MacLean (1919) and Fitting 

 (1926), it is very doubtful whether special protection against feeble 

 evaporation is needed at all. Dietrich (1925) made a painstaking 

 study of the transpiration of sun and shade plants in relation to 

 habitat. His results indicate that in this case also the accepted doc- 

 trine that increased surface of shade leaves is generally of service for 

 increasing evaporation needs further proof. Agreeing with Stocker 

 (1923), Maximov (1929), and Keller (1925), Dietrich showed that 

 transpiration per unit of area is less in shade plants than in sun plants. 

 The benefit of large transpiring surface of shade leaves is wholly can- 

 celed in many species by the decreased rate of surface transpiration. 



The necessity of promoting the passage of water into a saturated 

 atmosphere is still very uncertain, but everyone has observed the 

 deadly effect of atmospheric drought upon vegetation. At first the 

 plants wither; if water deficit continues, death ensues. In arid regions 

 morphological arrangements for reducing loss of water (c/. p. 125) are 

 especially common. Many of these protective mechanisms are now 

 genotypically fixed through heredity. Others, like leaf structure, size 

 of leaves, hairiness, root development, may be more or less modified 

 by the environment. Within limits set by heredity, the habitat has an 

 influence upon form, determining the degree of xeromorphy. 



This is accomplished especially by increasing the efficiency and 

 capacity of the mechanisms for increasing absorption and conduction 

 and for reducing the transpiration. However, it is difficult to obtain 

 an unequivocal numerical expression of this effect. The amount of 

 water given off per unit of surface of transpiring organs gives no infor- 

 mation whatever about the drought resistance of the plant. According 

 to Maximov (1929) and Szymkiewicz (1925), xerophytes often tran- 

 spire more than mesohygrophytes. Besides, transpiration varies not 

 only from species to species but also from individual to individual in 

 different habitat conditions and, indeed, from shoot to shoot and from 



1 However, MacLean (1919) was unable to demonstrate any guttation among 

 the hygrophytes of the Brazilian rain forest, and Shreve (1914o) found in the 

 rain-forest vegetation very few hydathodes. 



