334 LECTURE XV. 



With these preliminary remarks we will pass on to con- 

 sider the mechanics of growth, and we will begin by taking 

 the case of a single cell. 



I. The Mechanics of Growth. 



All young cells (omitting from consideration those which 

 do not possess a cell-wall) consist at first of a mass of proto- 

 plasm, including a nucleus, which is closely invested by a 

 cell- wall, the whole being saturated with water. When the 

 cell begins to increase in size, the protoplasm ceases to oc- 

 cupy the whole of the space enclosed by the cell-wall, that is 

 to say that the bulk of the protoplasm does not increase 

 as rapidly as the area of the cell-wall. Since the protoplasm 

 remains in close .connexion with the cell-wall at all points, 

 the result of the disparity between the growth of the proto- 

 plasm and that of the cell-wall is that small cavities, termed 

 vacuoles, make their appearance in the interior of the proto- 

 plasm, in which watery fluid, the cell-sap, collects. As the 

 cell further increases in size, the vacuoles also become larger 

 and usually coalesce so as to form a single large vacuole or 

 sap-cavity which is lined at the surface by the peripheral 

 protoplasm (primordial utricle), and which is traversed, for a 

 time at least, by protoplasmic strands connecting the primor- 

 dial utricle with a mass of protoplasm investing the more or 

 less centrally placed nucleus. This condition of the cell is 

 well shewn in Fig. 3 (p. 13). 



These are very briefly the phenomena which may be 

 observed in a growing cell : we have now to seek an explana- 

 tion of them. The point upon which we shall first fix our 

 attention is the growth in area of the cell-wall. It may be 

 stated at once that the growth i-n area of the cell-wall is due 

 to pressure exerted upon it by the cell-contents. In the early 

 stage of the development of the cell, when it contains no 

 vacuole, it is the protoplasm which exercises this pressure : in 

 the later stages of its growth, it is the cell-sap which exer- 

 cises it. We have already learned (p. 39) how a hydrostatic 

 pressure comes to be set up in a cell, and there is evidence to 



