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 
f Z aeration is secured. Hach 
ew IN jer. protoplast is thus in a 
Ee, SS somewhat intricate manner 
=e — 
in contact with the external 
air, though really situated 
perhaps deep in the tissues 
of a plant of large dimen- 
Fia. 89.—Snorron or a LENTICEL. : 
1, lenticel ; per, cork layer. s1ons. 
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 
