THE TRANSPORT OF WATER IN THE PLANT 81 
the liquid form. This happens when the hydrostatic 
pressure is very high at times in herbaceous plants, water 
being forced out at the tips of the leaves. It is not infre- 
quently seen 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 absorp- 
tion 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 aggrega- 
tions of corky cells which are developed in connection with 
the sheaths of cork that form part of the secondary tegu- 
mentary protective tissue of a thickened axis (fig. 39). 
They are not, however, so intimately connected with evapora- 
tion 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 protoplasts 
takes place at the so-called growing points, which are situated 
at the terminations of the twigs, and which give rise con- 
tinually to additional leaves and branches. The develop- 
ment of new material of this kind and of the new protoplasts 
6 
