6o METABOLISM 



Only plants that have a great development of water tissue can afford to be 

 lavish in their use of this water. Amongst our native plants, save the willow, 

 herbaceous forms only exhibit excretions of water when in full leaf, but in 

 tropical countries, where the water supply is abundant, it is especially noticeable, 

 e. g. in the long-stemmed lianas. If the excretion of fluid water by hydathodes 

 be rendered impossible in appropriate ways, the injection of the intercellular 

 spaces of the leaf takes place. In spite of the statements of Lepeschkin 

 (1902) that the activity of leaves is not affected by the infiltration of water 

 into them, we cannot but believe that the prevention of such an infiltration 

 by the activity of hydathodes must always be of service. 



Since probably all hydathodes are capable, under certain circumstances, 

 of absorbing water, they may be of service in this respect also. 



In cases where an actual excretion is not attained, bleeding-pressure may 

 be useful to the plant ; thus it is possible to demonstrate that such pressure 

 aids in the unfolding of buds in spring. 



The chief problem left for us now to consider is how does root-pressure aid 

 the ascent of sap in the plant ? We have already established the fact that by its 

 means water is pumped into the vessels ; whether it aids to any extent in the 

 transport of water the next lecture will show. 



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