Growth and Movement. 



105 



live part of the cell consists of the cytoplasm, a proto- 

 plasmic structure which occupies most of the cell cavity of 

 young cells ; the nucleus, which is suspended centrally in the 

 cytoplasm in young cells, but as the cell grows older may 

 have a lateral position 

 (Fig. 45); the leucoplasts, 

 small dense bodies which 

 multiply by division and 

 have special functions re- 

 counted in the next para- 

 graph; and the plasma 

 membrane, a special- 

 ized part of the cyto- 

 plasm which lines the 

 cell wall. (See Fig. 12 

 for details.) 



79. Functions of Cell 

 Organs. The plasma 

 membrane, cytoplasm, 



leUCODlastS, and nucleus A > longitudinal section through the root tip of 



an Onion ; B, successively older cells, i, from 

 Constitute the live part immediately back of the root cap where cell 



of the cell, and whatever d ,j vision g in f on : 2 an 3, older ceils, 



showing the modifications which I undergoes 



is done by the plant as with age. In i the cytoplasm fills the cell 

 i -i j cavity and the nucleus is relatively large. The 



a living body is accom- few h y eavy points indicate leucopksts 

 plished by one or more 



of these live parts or organs of the cell, which collectively 

 are termed the protoplast. We have noticed in our study 

 of roots that it is the plasma membrane which determines, 

 to some extent, whether certain substances shall pass to or 

 from the interior of the cell. It seems to be the guardian 

 of the cell and the inspector of all material interchanges. 

 It appears also to build the cell wall, and there is evidence 

 to show that it is the receiving organ for stimuli from the 



FIG. 4S . 



