﻿1903] NUCLEAR MEMBRANE 309 



Structurally the cell wall is entirely different from the ecto- 

 plast and tonoplast. Composed mainly of cellulose, it protects 

 the inner protoplast, gives rigidity to the whole cell, and allows 

 of a considerable internal osmotic pressure. As its method of 

 formation has nothing in common with that of the inner plas- 

 matic membranes, its presence will not form a part of the present 

 discussion. 



The two inner plasmatic membranes — which in the ordinary 

 vegetative cell constitute the outer and inner limiting layers 

 respectively of the primordial utricle — have the same general 

 structure, and their methods of formation are identical, so far as 

 we know. The ectoplast is present in every cell, no matter 

 whether the cell is surrounded by a cell wall or not. It is 

 very conspicuous in naked cells. In the ordinary cell, when 

 not in a state of plasmolysis, the ectoplast is always found in 

 close contact with the cell wall. As far as can be revealed by 

 the microscope, it is merely a differentiated film of the cytoplasm. 



The tonoplast is always to be found surrounding the watery 

 fluid of the vacuole. As young cells and most sporogenous cells 

 do not possess a vacuole, the tonoplast is not present. It is not 

 as constant a cell structure, therefore, as the ectoplast. It is, 

 like the ectoplast, merely a differentiated portion of the cyto- 

 plasm. Its formation may be followed readily by observing the 

 gradual development of the vacuole which is associated with the 

 growth of all vegetative cells. A complete series showing the 

 gradual formation of the vacuole may be observed in any grow- 

 ing region, such as root tips or growing points. Accompany- 

 ing the gradual accumulation of the watery contents of the 

 vacuole, the gradual differentiation of the cytoplasm coming in 

 contact with the cell sap becomes quite evident. Just what 

 changes in the minute structure of the cytoplasm take place in 

 formation of the membrane cannot be revealed by the micro- 

 scope. In this connection, however, Pfeffer says as follows: 



Every mass of watery fluid present in the protoplasm must be surrounded 

 by a vacuolar membrane to form a larger or smaller vacuole, while masses 

 of plasma which have escaped from the cell also become clothed by a plas- 

 matic membrane, and form large vacuolar bubbles in 'water, but not in plas- 

 inolysing solutions. From what has been said above, it follows that a plas- 



