566 ECOLOGY 



growth. They are necessary only in very small amounts and are readily 

 obtainable. The most luxuriant vegetation known is that of the humid 

 tropical forests, where transpiration often is very slight (sometimes 

 being almost negligible for days at a time), and there is no transpiration 

 in submersed vegetation; yet in neither of these instances do the plants 

 suffer from a lack of salts. Indeed, vegetative luxuriance varies in- 

 versely rather than directly with the transpiration. 



The possible advantages and the certain disadvantages of transpi- 

 ration. — The most probable advantage associated with transpiration is 

 in those plants which have a high turgor pressure (Part II, p. 336), 

 where it is a means of escape for an excess of water, the injection of air 

 spaces thus being prevented. It has been claimed also that, in sun- 

 shine, transpiration occasions a constant water renewal, which serves to 

 keep the leaf temperature near the optimum ; leaves exposed to sunshine 

 in saturated air may be some degrees warmer than are freely transpiring 

 leaves. These and other advantages, however, are to be regarded as in- 

 cidental. Except in water and in saturated air, transpiration is a neces- 

 sary companion of carbohydrate synthesis, since the very features (thin 

 expanded leaves, numerous open stomata, capacious air spaces) that 

 facilitate the latter also facilitate the former. One might conceive a 

 leaf so fashioned as to inhibit transpiration, but such a structure would 

 be valueless in synthesis. Transpiration, then, is necessary, whether 

 or not it is, as sometimes is believed, a necessary evil. Excessive 

 transpiration is the greatest danger to which plants are exposed, and 

 the harm that it entails certainly far exceeds any incidental good. 



The protective structures and activities of stomata. — The advantage 

 of closure. — The movements of guard cells commonly are interpreted 

 as having protective significance, since, were it not for transpiration, 

 stomata might remain open without harm. The closure of stomata by 

 night is regarded as advantageous, since carbohydrate synthesis ceases 

 upon light withdrawal, while suflScient o.xygen for respiration is easily 

 obtained. Closure in dry weather is regarded as useful, since at such 

 a time it is much more important that the water supply be conserved 

 than that synthetic activity be continued. In winter, when synthesis is 

 slight, closure is beneficial because transpiration is particularly harmful 

 by reason of diminished absorption. 



With the exception of light, the factors that increase transpiration also are the 

 factors that close the stomata. Light, therefore, would be a source of clanger, but 

 for the fact that the wilting it induces is followed, after a time, by stomatal closure 



