THE DIFFERENTIATION 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 BO IN per. 
dimensions. a=si4 AX 
Like the aquatic plant, aa 
the terrestrial one thus pos- = 
sesses a reservoir contain- 
ing an atmosphere which, Fra, 39.—SEction oF A LENTICEL. 
though its composition may 1, 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 
