22.] 



CHAPTER [. THE CELL 



CHAPTER I. 



THE CELL. 



93 



22. The Structure and Form of the Cell. In a fully 

 developed living cell the following three principal constituents 

 may be distinguished (Fig. 46 B C and D) : 



(1) A closed membrane, the cell-wall (/&), consisting generally of 

 a substance termed cellulose. 



(2) A layer of gelatinous substance, the protoplasm (j>), lying in 

 close contact at all 



points with the 

 internal surface 

 of the cell-wall ; 

 the protoplasm 

 gives the chemi- 

 cal reactions of 

 proteid. In it 

 lies a nucleus (fr), 

 in winch one or 

 more smaller 

 bodies, nucleoli 

 (//.) may generally 

 be distinguished. 



(3) Cavities, 

 one or more, in 

 the protoplasm, 

 termed vacuoles 

 (s), which are 

 filled with a wa- 

 tery liquid, the 

 cell-sap. 



The structure 

 of a ccenocyte is 

 essentially the 

 same as that just described, except that several (sometimes very 

 many) nuclei are present. 



The young cell presents a somewhat different appearance (Fig. 

 46 A}. At this stage the protoplasm occupies the whole cell- 

 cavity. But, in the subsequent development of the cell, the in- 

 crease in bulk of the protoplasm does not keep pace with the 



D 



FIG. 46. Cells and their structure. A Young cells from the 

 ovary of Symplioricarpus racemosus ( x 300) ; B cells from an 

 older ovary of the same plant ( x 300) ; C and D from the fruit 

 of the same plant ( x 100) ; h cell-wall ; p protoplasm : 7c 

 nucleus; fcfc nucleolus; s vacuole. In C there is a single 

 large vacuole, the whole of the protoplasm forming the parietal 

 layer. In D there are several vacuoles, and the nucleus lies in 

 a central mass of protoplasm connected with the parietal layer 

 by numerous strands. 



