450 The Physiology of Plants BOOK in 



ing etiolation was the subject of much speculation, but 

 it cannot so far be said to have been explained. In his 

 Vorlesungen Sachs suggested that the small size of etiolated 

 leaves is due merely to deficiency of food. Kraus and 

 Godlewski also attributed the phenomena to malnutrition 

 and interference with food. Batalin, in 1871, held that 

 the small size of etiolated leaves is due to a suppression 

 of cell division, but Prantl two years later demonstrated 

 the inaccuracy of this view by an actual computation of 

 the number of cells in the leaf under the two conditions 

 of light and darkness. Prantl suggested that the pheno- 

 menon is altogether pathological. De Vries, in 1879, held 

 that the action of light is the result of a direct influence 

 on turgidity, but other writers of the time did not find 

 turgidity increased with any regularity in darkness. In 

 1878 and again in 1886 Vines put forward the view that 

 the phenomena are due to a disturbance of the irritability 

 or the motility of the protoplasm of the growing cells, in 

 fact to a pathological condition, as Prantl had suggested 

 five years before. Pfeffer also held it to be not a mechanical 

 but a vital phenomenon, a reaction of the protoplasm to the 

 stimulus. An altogether different view, which has some 

 plausibility, was advanced by Palladin in 1893. He asso- 

 ciated it with the almost complete cessation of transpira- 

 tion which is noticeable in darkness, and pointed out that 

 the regions of greatest extension are just those which the 

 reduction of transpiration would cause to be over turgid. 



The question, however, seems still to await a solution, 

 for even the hypotheses of Prantl, Vines, and Pfeffer, do 

 little more than restate the problem. 



The rays of light which are concerned in restraining 

 growth were ascertained by Sachs in 1864 to be those oi 

 high refrangibility. He cultivated plants behind screens 

 of ammonio-cupric sulphate and potassium bichromate 

 solutions, and found that the latter cut off the rays con- 



