CELL-SAP. OSMOTIC PRESSURE. TURGOR 41 



chloroplast becomes almost dumbbell -shaped, the two polar areas 

 becoming more and more sharply delimited from the colourless equa- 

 torial zone, and gradually rounding themselves off to form the daughter 

 chloroplasts; as the latter move further and further apart, the colourless 

 zone becomes more and more indistinct and finally disappears altogether. 

 The author has found that the chloroplasts in the cortical cells of the 

 stem of Selaginelkt, illustrate every stage of transition between simple 

 constriction and the more elaborate form of division which involves 

 the early differentiation of a colourless equatorial zone. In the former 

 type of division the substance of the chloroplast long remains green in 

 the constricted region, which indeed does not lose its colour until it 

 has been drawn out into a fine thread. In the other type a colourless 

 zone is already visible at a time when the constriction is very slightly 

 marked (Fig. 6 b). It is an interesting fact, that in these cortical cells 

 of Selaginella the fission of the chloroplasts is never quite complete, 

 inasmuch as the daughter-chloroplasts formed at each division remain 

 joined together by a delicate colourless thread (Fig. 6 c, d) ; the chain 

 of chloroplasts, which is found in every old cell of the cortex, is thus 

 the product of the division of a single original chloroplast. 



5. Cell-Sap. 



It has been pointed out above that when an embryonic vegetable 

 cell begins to grow the protoplast soon develops cavities of varying 

 size, which become filled with a watery liquid, the cell-sap. As the 

 cell increases in size, these vacuoles usually coalesce to form a single 

 principal vacuole or sap-cavity. 17 The cytoplasm is separated from 

 each vacuole, or from the single sap-cavity, by the internal plasmatic 

 membrane or vacuolar membrane, which by de Vries is regarded as an 

 autonomous cell-organ [cf. p. 25]. 



The cell-sap usually consists of a clear, watery liquid, exhibiting an 

 acid reaction, and containing various inorganic and organic substances 

 in solution, especially the latter. It is the nature of the soluble con- 

 tents which determines the part played by the cell-sap of a given 

 protoplast in the economy of the particular cell, and in that of the 

 entire plant. 



The most widely distributed constituents of cell-sap are certain 

 organic acids (malic acid, oxalic acid, etc.), and their salts ; these, together 

 with other crystalloid substances, are responsible for the more or less 

 considerable osmotic pressure exerted by the cell-sap upon the peri- 

 pheral protoplasm and thus indirectly upon the cell-wall. The turgor 

 of the cell, which is the resultant of the osmotic pressure of the 

 protoplast and the elastic tension of the cell-wall, is of vital importance 

 to the plant in a variety of ways. Thus the comparative rigidity 



