378 The Physiology of Plants BOOK in 



searches so far described and their results must be very 

 incomplete. 



The forces which cause the movement did not receive 

 so close a study as the transformations concerned. Pfeffer's 

 great work on osmosis was accepted as affording an adequate 

 explanation of the passage from cell to cell till inquiry into 

 the nature of the membranes led to the recognition that 

 they are largely of the nature of semi-permeable membranes 

 and resist the passage of such substances as are under- 

 going translocation. It was found that the deposition of 

 starch, formed from the translocated sugar, is followed by 

 the passage of more sugar to the cells concerned in the 

 deposition, or, in other words, that there exists in such 

 parenchyma a tendency towards the maintenance of equi- 

 librium with reference to any constituent of the sap in the 

 cells. Osmosis does not conduce to this, for if the con- 

 centration of, e.g. sugar in any cell is lessened by its con- 

 version into starch, such cell will give up osmotically 

 more liquid to the adjacent cells than it will receive from 

 them ; its turgescence will diminish and the whole process 

 will be disturbed. Diffusion of the sugar was held to be 

 a probable contributor to the motive power till, in 1885, 

 De Vries showed that its rate is extremely slow, too slow 

 indeed to allow diffusion to be of much practical value. 



A theory was advanced by Brown and Escombe in 1900 

 which may perhaps lead to a fuller elucidation of the 

 problem. Recalling the observations of Gardiner on the 

 protoplasmic communications between contiguous cells, to 

 which we call attention elsewhere, these writers claimed 

 that the very fine passages through the cell wall render the 

 latter a membrane with minute apertures, which may be 

 compared with the membranes on which they based their 

 theory of the mode of entry of gases into plants through 

 the stomata. As we have already noticed this theory in 

 connexion with the absorption of carbon dioxide from the 



