166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
able concentration for the use of the chemistry of plants, the various 
needful chemicals brought up in extremely diluted form in the 
water imbibed by the roots. 
But this is not all. In producing this immense evaporation, the 
sun counteracts its own influence to unduly heat and scorch the 
delicate leaves. ‘Turning lquid water to vapor requires a very 
large supply of heat. So the sun-heat absorbed by the leaves is 
safely dissipated. Indeed, as in the human body, there is a rough 
uniformity of temperature preserved in plant leaves, and largely 
by the regulatory action of evaporation. Some plants, indeed, have 
automatic mechanisms which turn their parts toward the sun, 
or edgewise to its rays, according to requirement. ‘These plant mo- 
tions are well known, as we see them in the sunflower and the nastur- 
tium, and are, indeed, very common in the plant realm. 
Fourthly, the sun maintains a suitable temperature. Plant growth 
requires a state of temperature whose range is practically limited 
between 0° and 50° C. (82° and 122° F.). It is this state of affairs 
which the sun maintains constantly in the tropics, and through a 
part of the year in temperate and polar zones. Later we shall note 
some curious effects which temperature regulation may produce in 
plants. 
Such are the four great services of sun rays to plants, but in their 
response to these influences the plants exhibit a most interesting 
variety. Astonishing changes in growth and texture may be brought 
about merely by altering the temperature of environment, the dura- 
tion of sunlight, and the intensity and spectral quality of sun rays. 
Changes in the water ration, the chemistry of the soil, and the con- 
centration of carbonic acid gas in the air, also produce profound 
effects, but as these are but indirectly affected by the sun, we shall 
not discuss them, but turn our attention to the direct influences 
first mentioned. 
Col. Boyce Thompson has munificently established in Yonkers, 
New York, a laboratory splendidly equipped for the investigation of 
such effects, as well as for the study of plant diseases. In basement 
rooms there are provided cooling pipes and automatic regulators 
adapted to keep plants for as long as desired at definite tempera- 
tures and under powerful batteries of electric lamps adapted as a 
substitute for sun rays. The potted plants are mounted on little 
perambulators so that when the desired time of exposure in one 
temperature has elapsed, they may be removed to different tempera- 
ture surroundings. Instances of the curious results are shown in 
the accompanying illustrations. 
In another part of the laboratory is a glass-roofed hothouse. But 
the glass is not all the same. One part is tinged with violet, another 
