MUCILAGE. INTERCELLULAR SPACES 285 



Another important substance of the cell wall is 

 pectin, of which the middle lamella (the original thin 

 wall formed at cell division) is often composed. Pectin 

 is also formed in the cell walls (and cell contents) of 

 succulent fruits. The pentosans (see p. 275), which, 

 as we have seen, are characteristic of certain succulent 

 plants, also occur in lignified cell walls, being the 

 chief constituents of the " wood gums." The presence 

 of these and other carbohydrates which readily take up 

 water and become mucilaginous are the cause of the 

 ready formation of slimy substances from many cell walls 

 and cell contents on addition of water, for instance the 

 swelling of the middle lamellae of the medullary cells 

 of Fucus, the swelling of the surface cells of linseed 

 (p. 137), the breakdown into mucilage of the external 

 cells of the root cap (p. 290), and many other cases. 

 The process is the absorption of water by a carbo- 

 hydrate gel, quite comparable with the swelling of 

 gelatine considered in Chapter III (p. 53). 



Intercellular Spaces. Most of the living tissues of 

 the higher plants are interpenetrated by a system of air- 

 containing spaces (intercellular spaces) initiated by the 

 breaking apart of the cells along the plane of the middle 

 lamella. The air in these spaces is often called the 

 " internal atmosphere " of the plant. Its composition 

 agrees more or less closely with ordinary atmospheric 

 air, but except in tissues like the mesophyll of the 

 leaf (Fig. 10), which are constantly setting free oxygen 

 as the result of the photolysis of carbon dioxide, it 

 contains a smaller proportion of oxygen and a larger 

 proportion of carbon dioxide than atmospheric air, 

 owing to the respiration of the living cells, which use 

 up oxygen and set free carbon dioxide. The air in the 

 intercellular spaces is normally saturated or nearly 



