THE TKANSPOKT 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 Alcliemilla 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 nitration 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 



