6 STRUCTURE OF THE PLANT-CETL 



however, another layer of ehloroplasts appears Ijelonging to the 

 same ceU. We can thus hken each cell to an oblong box lined 

 along the whole of the inner surface with a thick viscid fluid 

 (the cytoplasm) in which are embedded the ehloroplasts, the 

 cavity of the box representing the vacuole (Fig. 3, Va.) with its 

 cell-sap. 



By watching the ehloroplasts, particularly in the elongated 

 cells of the midrib, they will be observed to exhibit movement 

 similar to that of the granules in the cells of the Spiderwort, and 

 here as there this is actually due to a flow of the cytoplasm. 



Fig. 3. — Two culls of the Canadian Pondwccd, one in surface view (Sji.), 

 and the other in optical section {O.S.). The ehloroplasts are shown 

 black. At the left a single chloroplast showing starch grains {St.). 

 Va., vacuole. 



Such a streaming movement probably takes place in the cyto- 

 plasm of all living cells, but in many cases it is so slow that 

 it cannot be demonstrated. By its means food-substances, etc., 

 are more tj^uickly distributed from one part of the cell to the 

 other than b)? mere diffusion. 



When leaves of the Canadian Pondweed, or those of other 

 plants, are kept in spirit for some little time, all the chlorophyll 

 is removed in solution. The ehloroplasts, though now colourless, 

 ha\'e undergone no change of shape, and are therefore merely 

 specialised parts of the cytoplasm \vhich held the chlorophyll. 



The green cells of all the higher plants contain numerous 



