34 VEGETABLE PHYSIOLOGY 



the lenticels, are present (fig. 39). The atmosphere conse- 

 quently enters the plant by these orifices and circulates 

 through the whole of the intercellular space system. As 

 nearly every protoplast abuts in part upon a channel of 



this system, its necessary 

 aeration is secured. Each 

 ,#er. protoplast is thus in a 

 somewhat intricate manner 

 in contact with the external 

 air, though really situated 

 perhaps deep in the tissues 



FIG. 39.-S E c~TioN OP A LENTICEL. of a P lant of lar g e dimen- 

 l, lenticel; per, cork layer. SlOnS. 



Like the aquatic plant, 



the terrestrial one thus possesses a reservoir containing an 

 atmosphere, which, though its composition may not be 

 exactly that of the exterior, yet contains oxygen for the 

 need of the protoplasts and serves as the medium by which 

 all surplus carbon dioxide is removed from them. 



This intercellular space system not only subserves the 

 purpose of the gaseous interchanges of respiration, but 

 ministers in two ways to the metabolic phenomena carried 

 out by the plant. It permits the access of the atmospheric 

 carbon dioxide to structures in the leaves which make it 

 available for the construction of food material. It further 

 is of great importance in helping to regulate the supply of 

 water to the cells. We have seen that a transport system 

 is differentiated which carries the water to them. This 

 transport system does not, however, remove it from them 

 subsequently. The protoplast can only get rid of water by 

 the process of evaporation, and as it constantly needs a new 

 supply, it must continuously exhale watery vapour to make 

 room for the incoming stream. Such evaporation takes 

 place into the intercellular spaces through the delicate cell- 

 walls which abut upon them. The intercellular reservoir 

 contains, therefore, an atmosphere which is charged almost, 

 if not quite, to saturation by aqueous vapour, and under 



