382 THE AMERICAN NA TURALIST. [VoL. XXXV III. 



Cytoplasm has surface contact with three conditions and in 

 each case there is present a delicate plasma membrane, colorless 

 and very finely granular, which is very' different in structure from 

 the cytoplasm within. The first of these three membranes is 

 the outer plasma membrane, which bounding the protoplast, is 

 consequently just inside the cell wall. This membrane is called 

 the " hautschicht " by the German botanists, a word for which 

 we have no exact equivalent, the term ectoplast more nearly 

 expressing the meaning than any other but for several reasons 

 not being very satisfactory. Since this outer plasma membrane 

 lies against a moist cell wall it is virtually surrounded by a film 

 of water. The functions of the cell wall in land plants and its 

 developmental history indicate a close relation to the demands 

 of the outer plasma membrane for a fairly uniform environment 

 of moisture, a matter which will be discussed in the last section 

 of these papers. 



The second form of plasma membrane surrounds the water 

 vacuoles in the cell. It is very common for the plant cell to 

 have a single large central vacuole containing the cell sap and 

 the membrane around this was named the tonoplast by DeVries 

 in 1885. DeVries believed that this vacuole reproduced itself 

 by fission with each cell division and consequently was a perma- 

 nent organ of the cell. It is, however, now well known that the 

 large central space containing cell sap is not different from other 

 vacuoles, indeed is frequently formed by the flowing together of 

 several small vacuoles as smaller soap bubbles unite in the froth 

 to form a larger one A vacuolar plasma membrane is of course 

 bathed by water since it holds the cell sap and its relation to a 

 moist surface is therefore more evident than in the case of the 

 outer plasma membrane. 



The third plasma membrane encloses the nuclear sap with the 

 protoplasmic nuclear elements chromatin, linin and the nucleolus. 

 This nuclear membrane was discussed in connection with the 

 nucleus of which it is generally considered a part, but as there 

 stated, the evidence largely indicates that it is cytoplasmic in 

 character, representing a reaction of this protoplasm to the fluid 

 nuclear sap formed around the chromosomes in the daughter 

 nuclei after each division (Lawson :c>3 a ). The nuclear sap 



