GENERAL CONSIDERATIONS 



21 



contractile fibers), and partly to different substances deposited within the 

 cell. Sometimes the last are present in such quantity that on first sight the 

 cell appears to consist only 

 of substances foreign to pro- 

 toplasm, as it is here defined. 



These cell contents vary a 

 great deal in kind : substances 

 which are taken up from, out- 

 side by the cell to be further 

 elaborated in it, substances 

 stored in the cell as reserve 

 material, substances formed in 

 the cell by its own activity to 

 be given out again under ap- 

 propriate circumstances, etc. 



In most plant cells the 

 protoplasm fills but a small 

 part of the cell body (Fig. 16). 

 Only those cells which lie close 

 to the growing tip consist en- 

 tirely of protoplasm. In their 

 growth the wall of the cell in- 

 creases in size much more rap- 

 idly than the protoplasm, and 

 as a result vacuoles are formed 

 filled with cell-sap. The nu- 

 cleus then lies embedded in a 

 mass of protoplasm, which is 

 connected by means of proto- 

 plasmic strands with a layer 

 inside the cell walls. The pro- 

 toplasm of such cells streams 

 back and forth within the cell 

 wall, carrying with it the gran- 

 ules embedded therein and oft- 

 times the nucleus as well. 



In the protoplasm of green 

 plant cells are contained spe- 

 cially differentiated chloro- 

 phyll bodies (cf. Figs. 25 and 

 31) to which these cells owe 

 their green color, and which are of very great importance in the vital activities 

 of plants (cf. page 23). Among the inclosures contained by the plant cell outside 

 the cell-sap, the starch granules are to be especially mentioned, since they repre- 

 sent the first visible product of the assimilative activity of the plant cell. 



Animal cells as a rule consist almost entirely of protoplasm and contain 

 foreign substances only in relatively small quantities ; they are therefore essen- 

 tially like young plant cells (Fig. 16). There are some animal cells also in 

 which the protoplasm is almost entirely displaced by foreign substances. This 

 is the case for example with fat cells in which the major part of the fat in 



P 



FIG. 16. Parenchyma cells from the middle layer of 

 the root-cortex of Frittilaria imperialis; longitudinal 

 section, after Sachs. A, portion of the section close to 

 the root-apex, very young cells, without cell-sap; B, 

 cells of the same layer about 2 mm. from the root- 

 apex; cell-sap (s) is forming in the protoplasm (p) ; 

 C, cells of the same layer 78 mm. from the apex. 

 The cell at the right above has been ruptured by the 

 razor; its nucleus (xy) is seen much swollen by absorp- 

 tion of water; k< nucleus; kk, nucleolus; h, cell-mem- 

 brane. 



