132 INTAKE AND DISTRIBUTION OF GASES 



The intercellular spaces are made by the' dissolution of the 

 middle lamella- of the cell-walls, particularly where three or 

 four cells come together, and a subsequent growing apart of 

 the cells at these places. Another mode of their formation 

 occurs also where in some instances the spaces are of uncom- 

 mon size, as in the hollow stems of grasses, thistles, Equisetums, 

 etc. In cases of this kind the large central cavity is formed by 

 the breaking down and disappearance of the interior funda- 

 mental tissue. The first method of intercellular space formation 

 is called schizogenous and the last, lysigenous. 



As a rule the intercellular spaces are larger in leaves than 

 in other parts of the plant body. The per cent, of the volume 

 of leaves occupied by intercellular spaces is different in dif- 

 ferent plants, varying all the way from 71.8 per cent, in Pistia 

 Texensis, a floating aquatic herb, to 3.5 per cent, in Begonia 

 hydrocotylifolia, a succulent creeping herb native to Mexico. 



A large volume of intercellular space in leaves is a response 

 to the need of them to provide the photosynthesizing or food- 

 constructing cells (see page 143) with carbon dioxide. Where 

 the habitat provides plenty of water the demand for large spaces 

 in the leaves can be satisfied without danger, as in Pistia; but 

 where water is hard to obtain and too great transpiration might 

 result from large intercellular spaces this provision for the circu- 

 lation and storage of carbon dioxide must be sacrificed to a 

 greater or less extent, as in the Begonia above mentioned, and 

 in plants in general of desert and other xerophytic habitats. 



In the stems and roots of plants growing under mesophytic 

 or average conditions of moisture the intercellular spaces are 

 very minute and can be made out only under close scrutiny 

 when good sections are prepared and studied under great magni- 

 fication. It seems from this that the large percentage of oxygen 

 in the atmosphere enables the gas to diffuse through the plant 

 body fast enough without much space being given over to diffusion 

 highways. The case is different, however, in water or marsh 

 plants, that is, plants under hydrophytic conditions, where 

 oxygen must be taken from its dilute solution in water, or must 



