10 Henry H. Dixon. 



Therefore 0-1817 cal. must be derived from the surroundings. In 

 this case it was found that the leaf was 1-84'' C. below its surroundings. 



In these experiments the energy needed for the vaporization is 

 supposed to be the latent heat of vaporization of a free liquid surface 

 at the temperature prevailing. It is evident that if this energj' is 

 used in doing the work of drawing up the water vaporized, as well as 

 in evaporating it, less water will be given off. We will return to 

 this point later. 



The transpiring cells of the leaf either are directly in contact 

 with the tracheae of the vascular bundles, as in the case of the 

 sheath cells, or groups of them abut on these latter. In any case 

 the water in the evaporating cells is directly or indirectly connected 

 with that in the tracheae. According to all observers the tracheae 

 of the leaves are normally filled completely with water and no bubbles 

 either of air or of water vapour are to be found in them during trans- 

 piration. There is then a constant supply of water in the tracheae 

 from which the losses of evaporation may be made good, if a higher 

 water vapour pressure exists in the tracheae than in the membranes of 

 the evaporating cells. With regard to the disposition of the tracheae 

 among the other elements of the leaf and their structure, it is to be 

 noted that they form together a continuous system throughout the blade 

 and petiole, which, while uninjured, is uninterrupted by the interposition 

 of cells, but trachea meets trachea in longitudinal series, so that water 

 will pass from the base of the petiole to any one of the extreme 

 terminations traversing tracheae alone, and need not at any place pass 

 through living cells. 



The most vexed problem of the Ascent of Sap is how the water 

 rises in the stem to fill the tracheae of the leaves. 



Botanists have sought solutions of this problem in two directions. 

 viz. (1) In the energy transformations taking place in the living 

 parts of the stem, namely in the cells of the wood and of the 

 medullary rays,, acting to raise the water, and (2) In the energy 

 transmitted and applied by means of the physical properties of the 

 conducting tracts and of the water stream itself, not necessarily 

 involving any special vital activity on the part of the cells of the 

 stem. Those hypotheses which belong to the first category may be 

 distinguished as the Vital and those of the second as the Physical 

 Theories of the Ascent of Sap. 



