CELL-WALL. 21 



it unfolds. Thus the young leaf is always guarded by the 

 ants, and no caterpillar or larger animal could attempt to 

 injure them without being attacked by the little warriors. 

 The fruit-like bodies are about one-twelfth of an inch long, 

 and are about one-third the size of the ants, so that the ant 

 bearing away one is as heavily laden as a man bearing a 

 bunch of plantains. I think these facts show that the ants 

 are really kept by the Acacia as a standing army to protect 

 its leaves from the attacks of herbivorous mammals and 

 insects." The above detailed account will give an idea as 

 to the amount of specialization entered into by some plants 

 in their endeavours to insure protection. Several other 

 plants belonging to widely separated families have arranged 

 on mutual terms with ants for protective purposes, suggest- 

 ing that the idea has originated independently in different 

 groups of plants. 



The substance of plants is not homogeneous, but consists 

 of minute portions, for the most part invisible to the unaided 

 eye. These minute fundamental parts are called cells, and 

 it is important to remember that every variety of form and 

 texture, as also the various colours, secretions, scents, etc., 

 depend entirely on division of labour and differentiation in 

 cells. The term cell implies a closed vesicle, and conse- 

 quently clearly expressed the idea entertained by the early 

 botanists as to the structure of this organ, but at the present 

 day many fundamental parts are known that are not covered 

 by the older definition ; nevertheless the old name is retained. 



A typical cell presents the following structure : (a) an 

 external protective membrane called the cell-wall, which is 

 composed of a substance formed by the protoplasm, and 

 having, at least when young, a constant chemical composi- 

 tion. This substance is known by the name of cellulose. 



