BOTANY. 289 



and solar radiation is absent at night. It is always positive. At a later stage 

 of advancement in plant physiology other natural sorts of radiation may need 

 to be studied. Radiation from artificial sources may now be employed in 

 experimental work, etc. 



The evaporating power of the air and that of radiation have been measured 

 in several ways, and the influences that they exert on plant transpiration have 

 been studied with corresponding instrumental records, in experiments carried 

 out at Tucson and at Baltimore. Progress has been made in the interpreta- 

 tion of records secured with the radio-atmometer, the black-bulb thermometer 

 and a photographic-paper actinometer, with reference to the actual, observed 

 influence of solar radiation on plant transpiration. Some comparisons have 

 been made with records from a pyrheliometer. 



Different kinds of plants differ in respect to their responses to solar radia- 

 tion, as far as transpiration is concerned, and the same plant individual 

 responds differently according to its stage of development and to the previous 

 treatment to which it has been subjected. As would be expected, internal 

 conditions, as well as other external conditions, are also markedly influential 

 on the responses to radiation. Healthy plants, which are not approaching 

 the wilting condition, generally respond to solar radiation relatively less than 

 does the black atmometer sphere, and they generally respond relatively more 

 than does the white atmometer sphere. But many details need to be con- 

 sidered. 



One interesting feature brought out in these studies is this, that the influence 

 of indirect solar radiation (radiation not received directly from the sun, but 

 coming from the rest of the sky, from the earth, etc.) is not at all to be neglected 

 in discussions of the influence of sunshine on plant transpiration. With no 

 clouds or other shade intervening between sun and plant the direct com- 

 ponent of solar radiation is of course predominant, but when the sun is 

 obscured by cloud the indirect component may become the more influential 

 of the two. With heavy cloud between sun and plant and with the rest of the 

 sky largely occupied by light cloud masses which serve as reflectors, the radia- 

 tion influence on the transpiration rate is still great in many cases. The two 

 components may be studied separately. 



It is hoped that these studies maj'^ eventually lead to the useful evaluation 

 of the sunshine feature of climate as a drying agent acting on plants, etc., 

 separating this important climatic character from the other drying influences. 

 Such an advance ought to illuminate many problems of physiology, physio- 

 logical ecology, agriculture, horticulture, forestry, and even hygiene and medi- 

 cine. Also, the insight that is being gained along these lines of experimenta- 

 tion is helping us toward a better understanding of the water relations of 

 plants and animals, and it is preparing for the time when it will be possible 

 to carry out physiological experiments on higher plants under controlled 

 conditions, so that some of the fundamental principles of plant control may 

 eventually be established. Such experimentation is not yet possible and is 

 greatly needed. 



During the last two years I have been assisted at Tucson as well as at Balti- 

 more, by Mr. J. D. Wilson, of the Johns Hopkins University. Much of the 

 work here referred to has been accomplished at the Desert Laboratory, the 

 facilities of which have been kindly placed at our disposal for long periods 

 by the Director of that laboratory. 



