114 ENVIRONMENTAL CONDITIONS. 



The amount of water in the transpiring organs appears also to be 

 important in determining the rate of water-loss. Thus, transpiration 

 is frequently checked in the daytime, with no apparent wilting and 

 no closure of stomata, but with a marked fall in foliar water-content, 

 while the evaporating power of the air is maintained or even increases 

 in magnitude.^ 



Second only to the nature and condition of the leaves, etc., the 

 evaporating power of the air exerts an enormous controlling influence 

 upon the rate of water-loss from plants. For any particular plant- 

 form it appears to be by far the most potent of all the climatic factors 

 affective above the soil-surface. The evaporating power of the air 

 is a compound factor, dependent itself upon three other factors, as 

 commonly considered — temperature, humidity, and wind velocity. 

 Its resultant effects are the summation of the partial effects brought 

 about through the influence of these three factors upon the vaporiza- 

 tion of water and the removal of the water-vapor from the evaporat- 

 ing surface. Evaporation, as a climatic factor, will be more thoroughly 

 considered in another place. 



The intensity of the sunlight and its quality are also potent factors 

 in the determination of transpiration from plants. While a certain 

 small proportion of the energy of the solar rays is made potential and 

 entrapped in the plant by the photosynthetic process, by far the 

 greater portion of that which is neither reflected nor transmitted 

 becomes potential in the water-vapor that escapes from the plant by 

 transpiration. Thus, intense sunhght with a good proportion of the 

 longer waves is markedly effective to increase the transpiration-rate 

 of plant organs whereon it falls. The color and structure of plant 

 organs have also to do with this, through the influence these exert on 

 reflection and transmission. With a smaller proportion of the greater 

 wave-lengths or with less intensity the effect is not so marked.^ 



Ecologists have classified plants, as to their ability to withstand 

 different degrees of Ught intensity, into those which thrive best in 

 shade, in bright sunshine, etc., and have given to these groups Greek 

 names, but inasmuch as there are all possible gradations in this power 

 of withstanding sunlight, and since there is as yet so httle information 

 of a quantitative nature bearing upon these matters, it seems advisable 

 here merely to emphasize the fact that one of the most important 

 influences of sunlight on plants is upon the rate of water-loss, and, 

 therefore, upon the water-requirement. 



Another form of water-loss from certain plants is the active excre- 

 tion of liquid moisture from nectaries and other superficial glands, 

 such as water-pores, etc. The process by which this is brought about 



1 Livingston, B. E., and W. H. Brown, Relation of the daily march of transpiration to variations 

 in the water-content of foliage leaves, Bot. Gaz., 53: 311-330, 1912. — Shreve, Edith B., The 

 daily march of transpiration in a desert perennial, Carnegie Inst. Wash. Pub. No. 194, 1914. 



^ Livingston, B. E., Light intensity and transpiration, Bot. Gaz., 52: 418-438, 1911, 



