174 The Anatomy of Plants BOOKII 



of certain substances principally or entirely protein, actually 

 precipitated on the protoplasmic films which bound the free 

 surfaces. These are the well-known ' plasma membranes ' 

 associated with this theory. 



Pfeffer's theory attracted much attention, and met with 

 general acceptance for a time. It was, however, passing 

 out of favour at the end of the century. 



A different view of the external surface of the proto- 

 plasm and its relation to the cell wall, was taken 

 by Chodat and Boubier in 1898 ; it was based on actual 

 experiment, but scarcely received as much attention as it 

 deserved. When these observers set up a very gradual 

 plasmolysis of the cell the protoplasm did not wholly leave 

 the walls, but remained attached to them in places by thin 

 plates which extended between the two. The direction 

 of these plates did not always correspond in contiguous 

 cells, so that they were not formed of communicating 

 threads passing through the wall. The plasmolyzed proto- 

 plasm remained so for a variable time, united to the wall 

 by more or fewer bands of ectoplasm. 



The authors explained the formation of the bands or 

 threads by the suggestion that the ectoplasm is viscous 

 and adheres to the membrane, or else that the external 

 surface or layer of the ectoplasm passes insensibly into the 

 substance of the cell-wall, and so, by this close union and 

 progressive change at the plane of contact, takes part in the 

 formation of the new lamellae of the cell-wall as the latter 

 gradually thickens. 



A very different theory of the wall or limiting layer of the 

 vacuole was brought forward in 1886 by De Vries. He 

 advanced the view that the young cell contains among the 

 products of its organization certain structures to which he 

 gave the name of Tonoplasts, small bodies which secrete 

 strongly osmotic substances in their interior, and which 

 consequently swell up very considerably, becoming recog- 



