76 VEGETABLE PHYSIOLOGY 



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 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 aggrega- 

 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 

 \/tr j^ 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 



