64 



ELEMENTARY BOTANY. 



when it disappears the cell is dead. It is very complex both 

 in structure and composition. Its constituents are albumin- 

 oids (containing oxygen, hydrogen, carbon and nitrogen), a 

 variable amount of water, and a small quantity of ash or 

 mineral constituents. 



4. The protoplasm secretes the cell-wall. This consists of 

 cellulose, which is composed of carbon, hydrogen and oxygen 

 (but no nitrogen), also water and ash, or mineral constituents. 

 The cell-sap is a watery fluid containing plant food in solu- 

 tion. It fills the apparent vacant spaces, called vacuoles, in 

 the cell and also permeates the protoplasm. The cell has no 

 wall or covering in the earlier stage of its existence. In a few 

 cases none is ever formed, at least in the vegetative stage or 

 that preceding the reproductive period. The free protoplasm 

 can in such case swim about, as the so-called Swarm-spores 

 (Fig. 96) are seen to do. Or it may creep about, as do the 



Fig. 97. 



Pig.! 



Slime-Moulds. When the protoplasm is enclosed in a wall it 

 may yet manifest movements. In some cases there are slen- 

 der streams moving about in various directions, mostly from 

 the central to the parietal portions and in the reverse direc- 



