THE DIFFEBENTIATION OF THE PLANT-BODY 39 



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 proto- 

 plast is thus in a somewhat intricate manner in contact with 

 the external air,though really 

 situated perhaps deep in the 

 tissues of a plant of large 

 dimensions. 



Like the aquatic plant, 

 the terrestrial one thus pos- 

 sesses a reservoir contain- 

 ing an atmosphere which, Fre< SO.-SECTTON OF A Lmracm. 

 though its Composition may I, lenticel ; per, cork layer. 



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 

 ordinary atmospheric conditions this is being continually 

 exhaled as long as an excess of water is passing through 



