EVAPOKATION OF WATER 419 



each communicating with the system of intercelhilar spaces, 

 and having two guard-cells surrounding it. Through these 

 stomata the watery vapour evaporated into the intercellular 

 spaces passes to the external air, the quantity escaping heing 

 regulated b}^ the degree to which the guard-cells are approxi- 

 mated. We have thus a copious evaporation taking place from 

 the surface of the leaves, which plays an important part in 

 causing the flow of water through the plant. This evaporation 

 is known as transpiration and must be discussed more fully 

 later. 



Little or no evaporation takes place from the surface of the 

 epidermis of the leaves, the cells of which have their outer 

 walls generally strongly or weakly cuticularised, the cuticle 

 offering considerable resistance to the passage of water, or 

 watery vapour, in either direction. 



The escape of water is thus Fig. 



mainly brought about by evapo- 

 ration into the intercellular spaces 

 of the leaves. It is not con- 

 fined, however, to these, but takes 

 place generally- into all the inter- 

 cellular spaces, which form a system 

 of minute channels permeating the 

 whole plant. Tlie actual escape 

 from the plant takes place wherever Fi-j. 11^3. Section of leaf of Beta. 

 tbprp flvp ^tomatn a^ wpII tbati>^ '-. Epidermis. /jX Palisade tissue. 



tnere aie siomaia, as^\en. mat is, ^^ spougv parenchyma with 

 from the young green twigs as if^rge intercellular spaces. 

 from the leaves themselves. It is 



supplemented in some cases by an actual excretion of water as 

 such, carried out by means of the water glands we have seen to 

 occur in some plants at the ends of some of the veins of the 

 leaves. Wlien the hydrostatic pressure is very high at times in 

 herbaceous plants, water may be forced out of the tips of the 

 leaves without the intervention of glands. 



The stream of water thus passing through the plant has a 

 very important influence upon its development. This is evident 

 from the consideration that a large part of the food of the plant 

 is absorbed b^- means of this stream, the water containino- 

 minute proportions of the various nutritive salts which exist in the 

 soil. The quantity passing is further correlated with the amount 

 of leaf surface which the plant presents ; where there is a larcre 

 leaf area there is copious transpiration ; this necessitates a larger 

 path for the ascending stream and a consequent development of 



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