THE MOLECULAR FORCES IN PLANTS. 223 



is kept dry, while the bulb of the other is wrapped with wet 

 linen. The results of my investigations are detailed in my Beitriiye 

 zur Theorie des Wurzeldrucks, Jena, 1877, p. 77. The cause of the 

 increase in transpiration under illumination is to be sought on the 

 one hand in the heating effect of light rays penetrating into the 

 plant, while on the other hand it may, in many cases, be still 

 further materially intensified by the widening of the slits of the 

 stomata brought about by access of light. 



This last factor is also of the utmost importance in explaining 

 the following simple experiments. We take two narrow-mouthed 

 bottles filled with water. In one we place a large twig of Tilia 

 grandifolia, in the other a shoot of Salix fragilis. We cover the 

 water in both with a layer of olive oil, and weigh. Both are then 

 exposed to diffuse daylight, and after a time we weigh again, and 

 so determine the loss by transpiration. I found that a large Tilia 

 shoot lost in 1J hours, only O5 gr. of water, while willow twigs 

 under the conditions described transpired very vigorously. Ex- 

 posed to direct sunlight, the Tilia twig gave off in 1J hours 45 gr. 

 of water. In Tilia the stomata are only widely open in direct sun- 

 light, whereas they close in diffuse light. In Salix the stomata 

 remain open even in diffuse light, and the shoots consequently 

 transpire very vigorously even under these conditions. 



That the stomata of the lime do really react very energetically 

 to changes of illumination, can easily be proved by the Cobalt 

 test. Cut twigs of Tilia grandifolia are put with their lower ends 

 in water, and exposed, some of them to direct sunlight, the rest to 

 feeble diffuse daylight. After two hours it is shown by the Cobalt 

 test (see 82) that the leaves of the former turn the Cobalt paper 

 in contact with their under surfaces strongly red, while those of 

 the latter, exposed to diffuse light, redden the paper only to a 

 slight extent. With the help of the Cobalt test I found also that 

 leaves of Tilia twigs, exposed to direct sunlight without access of 

 water, rapidly closed their stomata. 



The stomata of Aspidistra elatior and Ficus elastica, according 

 to Stahl's observations, also react unusually energetically to 

 changes of illumination. In direct sunlight they open their sto- 

 mata widely, while in not too bright diffuse daylight, even at a 

 high temperature (30 C.), the stomata remain closed. Aspidistra 

 and Ficus consequently transpire very feebly in diffuse daylight, 

 very energetically in direct sunlight. If a pot plant of Ficus 

 elastica, which has been sunned for about an hour, is weighed, 



V' 



QNIV 



