No. 449.] STUDIES ON THE PLANT CELL. 



necessitates the development of a vacuole which becomes 

 bounded by the nuclear membrane. The nuclear membrane 

 in some cases at least differs from a vacuolar membrane in 

 being easily distinguished from the surrounding cytoplasm as 

 a definite film. 



The structure of all the plasma membranes is much the same 

 as far as the microscope may determine. The protoplasm is 

 dense, colorless and filled with very minute granules (micro- 

 somata). There are no large inclusions such as plastids, parti- 

 cles of food material (starch, proteids, oils, fats, etc.), mineral 

 matter or waste products. These are all held well within the 

 cytoplasm between the outer plasma membrane and the vacuoles. 

 There is good reason to believe that the substance of all plasma 

 membranes is much the same since they perform very similar 

 activities both in relation to the fluids that bathe them and also 

 because their substance in certain cases becomes the proto- 

 plasmic basis of cellulose walls. These resemblances are well 

 established for the outer plasma membrane and that which sur- 

 rounds the vacuoles. Thus, the capillitium of Myxomycetes 

 (Strasburger, '84) is formed from the plasma membranes around 

 the vacuoles after the same method as a cell wall from the outer 

 plasma membrane. And again, during cleavage by constriction 

 (see section II) in the plasmodium and sporangium of the molds 

 (Harper, '99 and : oo, D. Swingle, : 03), vacuoles fuse with cleav- 

 age furrows from the outer plasma membrane to form a common 

 membrane which surrounds each spore mass and secretes a wall, 

 thus showing identity of function and structure. The resem- 

 blances are less conspicuous for the kinoplasm of the nuclear 

 membrane, only appearing indirectly with certain events of cell 

 division (the formation of the cell plate) which will be discussed 

 in the next section of the paper. The evidence indicates that 

 the three plasma membranes are all kinoplasmic in character, a 

 generalization of some importance since it offers explanations of 

 many peculiar cell activities to be described later. 



Since all plasma membranes have these common characters it 

 may well be questioned whether an elaborate terminology is 

 justified for structures so closely related. The terms ectoplast 

 and tonoplast seem undesirable since they were meant to indi- 



