THE ASSIMILATING CELL. 



19 



" rotation." In fact, it is only in the adult cell that these are to 

 be seen. The cells of a very young leaf or a rudimentary organ 

 of any kind present very different features ; in the first place, the 

 protoplasm almost entirely fills the cell-cavity, and the nucleus is 

 situated in the geometrical centre of the cell. Moreover, chloro- 

 plasts do not, as a rule, appear as such in the cells of organs 

 which will ultimately become green until those organs have been 

 exposed to light (there are a few'^xceptions to this statement — 

 e.g., the seed leaves of Pinus and the green layer in the cortex of 

 stems just internal to the 'cork), but are replaced by structures 

 known as plastids or leucoplasts, which are, so to speak, chloro- 

 plasts in which as yet no chlorophyll has been formed (see Fig. 4). 



==k^mmmf^mM^ms'''mir 



Fig. 4.^You>;g Cells prom a Koot-tip. — The cytoplasm fills the cell- 

 cavity, and the nucleus is a, relatively large structure. The small 

 oval bodies are plastids. 



The protoplasm of such a young cell does not show the 

 " streaming " movement seen in some older cells, and the cell-sap 

 is small in amount, does not at first form vacuoles in the proto- 

 plasm, but exists in it somewhat as water does in the meshes of 

 a sponge. The cell-wall is very thin, and gives the characteristic 

 ^'blue" reaction for cellulose when treated with iodine and 

 sulphuric acid ; acetic acid will cause a shrinking away of the 

 protoplasm from the wall, but not to the same extent as in older 

 cells, and the nucleus will, under these conditions, show the 

 punctate appearance before mentioned. The plastids are turned 



