CH. l] SPIROGYRA. 7 



notice that cell-division in this sense does not necessarily 

 mean that the cell is actually split into two free halves : 

 in the case of Spirogyra, and in growing plants generally, 

 the original cell is simply divided into two compartments 

 which increase in size and may again divide. It follows 

 from this manner of growth that a Spirogyra as it grows 

 comes to consist of more and more cells. 



\ , \ 



p. 71. C. p.U. 



FIG. 2. 



A CELL OF SPIROGTKA. 

 c, the spirally wound cbloroplast. 



p. u, the protoplasm lining the cell (primordial utricle), 

 n, the nucleus suspended by protoplasmic ropes. 

 p, a pyrenoid with numerous small starch grains. 



Each compartment of the plant is a good example of 

 the perfect vegetable cell. It has a cellulose wall often 

 coated outside with a layer of slimy material ; the cavity 

 of the cell is lined with a coating of protoplasm, inside 

 which is a large vacuole taking up nearly all the room 

 inside the cell. The fluid in the cell cavity is called 

 cell-sap, and is a very dilute solution containing certain 

 salts, vegetable acids, sugar and tannin. In the proto- 

 plasm a certain part is differentiated from the rest into 

 what is called a chloroplast that is to say a piece 

 of protoplasm coloured green with the substance chloro- 

 phyll. When a green leaf or a Spirogyra plant is put into 

 spirit or ether it becomes colourless because the chloro- 



