224 PHYSIOLOGY OF NUTRITION. 



and then again weighed after exposure for two to four hours to 

 direct sunlight, a considerable loss from transpiration is observed. 

 If now the plant is placed at the back of a room with a north 

 aspect, and we begin a new experiment after the plant has been 

 exposed for about an hour to the changed conditions, then the 

 weighings indicate very slight losses by transpiration. In these 

 experiments loss of water from the pot and soil may be prevented 

 by covering the soil with tinfoil, and wrapping the pot with a 

 white cloth, or, for greater safety, we may proceed as in the 

 experiment illustrated by Fig. 81. 



We cut off twigs of Salix fragilis and Tilia grandifolia. The 

 twigs, which must be approximately of the same weight, are 

 brought on to a table in a room with a north aspect. They are 

 not supplied with water. Repeated weighings during one to two 

 days show that the willow twig transpires much more strongly 

 than the lime twig. Therefore also the leaves of the willow are al- 

 ready dry when the lime leaves still contain a comparatively large 

 quantity of water. If we examine withering leaves of lime and 

 willow (leaves of, e.g., Cyperus alternifolius behave quite like the 

 latter) by the Cobalt test, we find the stomata of the lime closed 

 and those of the willow open. The lime and many other plants 

 are able to regulate the intensity of their transpiration. They 

 close their stomata when they are threatened with danger of 

 withering. Willows cannot do this ; their leaves consequently 

 dry very rapidly when supply of water is prevented. 



To make experiments in winter which teach that withering 

 leaves close their stomata, we employ species of Tradescantia from 

 the hothouse. Freshly cut leaves redden Cobalt paper brought 

 against their lower surface, while slightly withered leaves can 

 no longer do this. If shoots of Cyperus alternifolius and leaves 

 of Aspidistra, which are also always available in winter, are cut 

 and left in diffuse light, without supply of water, the former 

 rapidly wither because their stomata cannot close, the latter for a 

 long time keep fresh since their stomata are shut. The Cobalt 

 test may also be conveniently used here to gain information as to 

 the state of the stomata. 



Specimens of Cucurbita or other plants are investigated as to 

 their transpiration by means of the apparatus represented in Fig. 

 79, or that in Fig. 82. They are first left standing on a balance 

 for half an hour at rest, and the loss by transpiration is deter- 

 mined. Then they are violently shaken for a few seconds, and it 



