42 THE RELATION OF DESERT PLANTS TO 



determining whether such treatment accelerates or retards growth and 

 the absorption of salts. As far as the writer is aware, no experiments 

 have been carried out with sufficient accuracy to make their results 

 applicable here in more than a general way. 



While a certain amount of transpiration may be necessary for plant 

 life in general, it is evident that this does not need to be very great, 

 first, from the fact that the most luxuriant vegetation occurs in the 

 humid tropics and in greenhouses, where transpiration is relatively low, 

 a point brought out by Reinitzer, Haberlandt, and others, and second, 

 from the mere fact of existence of the xerophilous types in which, as 

 is well known, the amount of transpiration is kept very low by struc- 

 tural modifications. It is probably safe to assume that by far the 

 greater portion of the transpiration of desert plants is only a neces- 

 sary evil. The forms here found are so adapted to xerophytic condi- 

 tions that their transpiration is reduced to as low a figure as is com- 

 patible with the exposure of sufficient surface of moist membranes to 

 secure the necessary carbon dioxide for photosynthesis. 



The conditions affecting transpiration in any given plant are, of 

 course, the evaporating power of the air, the supply of water available 

 to the roots, and, to some extent, physiological responses of the 

 leaves, such as the stomatal responses, to changes of light, tempera- 

 ture, etc., and the nyctitropic movements of the leaves themselves. 

 Since the water relation is of paramount importance in all plants, and 

 especially so, as has been already noted, in the forms inhabiting the 

 desert, transpiration becomes probably the most important phenomenon 

 in determining the nature of the vegetation in these regions. There- 

 fore, attention was largely confined during these investigations to 

 measurements of the effect of the three factors mentioned above as 

 controlling transpiration. The results will be given under the two 

 headings, ''Measurements of transpiration" (including some discussion 

 of the effect of nyctitropic movements and regulatory phenomena), 

 and "Water requirements." 



SOME MEASUREMENTS OF TRANSPIRATION; A NEW METHOD FOR STUDYING THE 

 PHYSIOLOGICAL REGULATION OF THIS FUNCTION. 



As has been emphasized above, in order to obtain measurements 

 of the transpiration rate which will most nearly approximate the condi- 

 tions in naturally growing plants it is necessary to take these measure- 

 ments in the open air, without inclosing the plant in a chamber. This 

 is to take account of the effect of air currents which have been shown, 

 especially by linger (1861), to exert great influence on evaporation 

 and transpiration. It is further necessary not to injure the plant in 



