494 The Physiology of Plants BOOK i 



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

 the cells of the lower side of the pulvinus to the intei 

 cellular spaces. That this is caused by a modification c 

 the permeability of the protoplasm and not by any activ 

 contraction as was supposed by Cohn in 1876, was advance 

 by Vines in 1886 ; he based his opinion on Darwin 

 observation that loss of turgidity is an accompaniment ( 

 aggregation in the cells of the tentacles of Drosera as the 

 curve. 



Kraus observed, in 1879 and 1880, that both heliotropi 

 and geotropic stimulation cause curious changes in i\ 

 normal metabolism of the different parts of the plant ; h 

 was able to notice an increased production of reducin 

 sugar in the cells of the lower side of a shoot placed hor 

 zontally before the curvature had begun ; this diminish^ 

 again during the period of curving. Variations in th 

 quantity of free acid on the two sides were also noticec 

 These changes are also shown by stems which from the: 

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

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

 stimulus. 



Researches have already been described which show tha 

 response to stimulation does not always take the form c 

 movement or curvature. Some curious effects of the ir 

 fluence of a unilateral light on the constitution of planl 

 call for notice. In 1876 Leitgeb showed that it induce 

 the permanent dorsiventrality of the germ filaments c 

 protonemata growing from the spores of certain Marchar 

 tiaceae. Pick found in 1882 that twigs of Biota oriental? 

 develop dorsiventral structure under the same conditions 

 Czapek, in 1898, showed that Marchantia gemmae, rota 

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

 to weakly plants of radial structure. In 1886 Vochtinj 

 showed that the dorsiventrality of the flowers of Epilobiun 

 angusti folium, Hemerocallis fulva, and Clarkia pulclidla 



