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 leiicof lasts, 

 small dense bodies which 

 multiply by division and 

 have special functions re- 

 counted in the next para- 

 graph; and the plasma 

 yneinbrane, 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, 



leUCOplastS, and nucleus '^^ 'ongi'udihal section through the root tip of 



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

 immediately back of the root cap where cell 

 division is going on ; 2 and 3, older cells, 

 showing the modifications which i undergoes 

 with age. In i the cytoplasm fills the cell 

 cavity and the nucleus is relatively large. The 

 few heavy points indicate leucoplasts. 



FIG. 4S. 



constitute the live part 

 of the cell, and whatever 

 is done by the plant as 

 a living body is accom- 

 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 



