494 The Physiology of Plants BOOK in 



was shown by Pfeffer to be a passage of the water from 

 the cells of the lower side of the pulvinus to the inter- 

 cellular spaces. That this is caused by a modification of 

 the permeability of the protoplasm and not by any active 

 contraction as was supposed by Cohn in 1876, was advanced 

 by Vines in 1886 ; he based his opinion on Darwin's 

 observation that loss of turgidity is an accompaniment of 

 aggregation in the cells of the tentacles of Drosera as they 

 curve. 



Kraus observed, in 1879 and 1880, that both heliotropic 

 and geotropic stimulation cause curious changes in the 

 normal metabolism of the different parts of the plant ; he 

 was able to notice an increased production of reducing 

 sugar in the cells of the lower side of a shoot placed hori- 

 zontally before the curvature had begun ; this diminishes 

 again during the period of curving. Variations in the 

 quantity of free acid on the two sides were also noticed. 

 These changes are also shown by stems which from their 

 age are not able to curve. As they occur before the bending 

 commences it is clear they are due to the action of the 

 stimulus. 



Researches have already been described which show that 

 response to stimulation does not always take the form of 

 movement or curvature. Some curious effects of the in- 

 fluence of a unilateral light on the constitution of plants 

 call for notice. In 1876 Leitgeb showed that it induces 

 the permanent dorsiventrality of the germ filaments or 

 protonemata growing from the spores of certain Marchan- 

 tiaceae. Pick found in 1882 that twigs of Biota orientalis 

 develop dorsi ventral structure under the same conditions. 

 Czapek, in 1898, showed that Marchantia gemmae, rota- 

 ting on a klinostat for two to three months, gave rise 

 to weakly plants of radial structure. In 1886 Vochting 

 showed that the dorsiventrality of the flowers of Epilobium 

 angusti folium, Hemerocallis fulva, and Clarkia pulchella, 



