THE PLANT CELL 99 



skeleton or framework of the plant body, and in the 

 higher plants a considerable part of the structure (in 

 trees the great bulk of the structure) is composed of 

 the walls of cells which have died, so that only the 

 wall is left. These walls are often very thick and 

 hard, growth in thickness and the deposition of sub- 

 stances other than cellulose having taken place before 

 the protoplasm of the cell died and disappeared. 

 Another feature of the tissues of the higher plants 

 (land plants) is the existence of a system of air spaces 

 (intercellular spaces) between the cells. These are 

 made by the separation of the walls of certain adjacent 

 cells from one another, and penetrate the plant, com- 

 municating with the outer air and forming the channels 

 by which gases, such as water vapour, carbon dioxide 

 and oxygen, produced or used by the living cells, diffuse 

 from these to the air outside, or vice versd. 



THE PLANT CELL. 



The first cells of organisms to be described were those 

 of the cork tissue of plants, which were seen by the 

 Englishman Hooke in 1667 with the compound micro- 

 scope that was devised about the middle of the seven- 

 teenth century. Hooke compared these cork cells with 

 the " cells " of honeycomb. Thus it was the walls 

 enclosing cavities to which the term was first applied. 

 Little accurate knowledge of the contents of these cavities 

 was obtained for nearly two centuries. But in 1831 

 the English botanist, Robert Brown, discovered the 

 cell nucleus, and in 1846 the German botanist, von 

 Mohl, identified the semi-transparent viscous substance 

 formed within many of the " cells " of the adult higher 

 plant as the actual living substance of the organism, 



