6i 



We have also seen when transpiration is not active, the stomata 

 are not required to be open, and hence they are found to be closed. 

 An adequate amount of transpiration is very important to the 

 plant, because it is due to this that a supply of inorganic substances 

 are absorbed in solution. These substances we saw could only be 

 taken up in very small quantities, and since a very large supply of 

 them is required in the plant, it follows, as a necessary consequence, 

 that a very considerable amount of transpiration is needed to 

 supply an adequate amount of nutrition to the plant. Further, as 

 we shall see, the formation of starch from the Carbon Dioxide, 

 absorbed by the cells of the leaf, independently of the stomata, is 

 dependent on the absorption of certain inorganic salts, as e. g. those 

 of Potassium, by the roots ; and hence it is necessary that while the 

 process of absorption of Carbon Dioxide is going on actively, 

 under the conditions of bright light and warmth, a large quantity 

 of these salts should be absorbed at the same time ; and this can 

 only take place by means of sufficient transpiration at the leaves, 

 through the widely open stomata. 



As to the method of transference of fluid from the vessels of the 

 woody core to the different parts of the plant, or in other words, 

 as to the course of the rising sap, different views have been enter- 

 tained. The chief structural elements composing the central 

 core of the wood or prosenchyma, are vessels and fibres to- 

 gether with ordinary parenchyma cells ; and the question arises, 

 does the water rise uniformly through all of these, or 

 only through some of them, and if so, through which? At 

 first sight, the cavities of the vessels would appear to be the 

 most natural channel for the water, since they are open from 

 one end of the plant to the other, and the old idea was that 

 in them the water was conveyed, that they acted like blood- 

 vessels, and contained fluid. In spring, the cavities of the 

 vessels do contain water, but as summer comes on and the 

 leaves expand the amount of water gradually decreases, and 

 they become filled with air, and when transpiration has really 

 set in the whole cavity is occupied by air, so that in summer, 

 when transpiration is most active, and consequently the amount 

 of fluid passing to supply the loss which it occasions is greatest, 



