76 VEGETABLE PHYSIOLOGY 
geen in the case of grasses, the edges or apices of whose 
leaf-blades may show drops of liquid standing upon them 
in the early morning. Similar drops are often to be seen 
on the surfaces of the leaves of Alchemilla when they have 
ceased to transpire during the night, while the absorption 
of water by the root has continued actively. The escape of 
liquid in this way is due to a filtration similar to that by 
which the water is forced into the woody elements of the 
stele of the root, as previously described. 
A subsidiary mechanism allowing the escape of watery 
vapour from the cortex of stems and roots is provided by 
the lenticels. We have seen that these are loose agerega- 
tions of corky cells which are developed in connection with 
the sheaths of cork that form part of the secondary 
tegumentary protective tissue of a thickened axis (fig. 39). 
They are not, however, so intimately connected with evapo- 
ration as the stomata, probably being more concerned with 
the aeration of the tissue. 
The stream of water thus passing through the plant 
has a very important influence upon its development. We 
have seen how important a factor in its growth is the 
maintenance of a condition of turgescence, which in turn 
depends on the constant absorption of water to take the 
place of that removed by evaporation. The quantity pass- 
ing is correlated with the amount of leaf surface which the 
plant possesses; where there is a large leaf area there is 
copious transpiration ; this necessitates a large path for the 
ascending stream, and a consequent development of the 
axial portions of the plant. 
The greatest increase in the number of the proto- 
plasts takes place at the so-called growing points, which are 
situated at the terminations of the twigs, and which give 
rise continually to additional leaves and branches. The 
development of new material of this kind and of the new 
protoplasts which they contain is largely dependent upon 
another feature of the water supply to which attention 
has already been called. A considerable part of the 
