WATER-CONDUCTION. EXPERIMENTAL EVIDENCE. 317 



close correlation which exists between the requirements of particular 

 organs in respect of water-conduction on the one hand, and the number 

 and diameter of their tracheal elements on the other. This correlation 

 will be referred to again on a subsequent occasion. As regards the 

 histology of the walls of water-conducting elements, special reference 

 may once more be made to the bordered pits ; the significance of these 

 peculiar structures becomes partially comprehensible, if one assumes 

 that water moves in the cavities of the cells to which they belong. 

 This point will also receive more attention later on. 



At this stage it will be desirable to summarise in half a dozen 

 sentences those facts concerning the conduction of water which may be 

 regarded as definitely established : 



1. Vessels and tracheides normally contain both air and water, the 

 relative amounts of the two substances varying according to the season 

 and the time of day. 



2. The pressure of the air in the tracheal elements varies, but is 

 frequently very low. 



3. The injection of water into the vessels and tracheides is effected 

 by osmotic forces, which reside chiefly in the root-system. 



4. The partial removal of water from the vessels and tracheides is 

 due to the osmotic " suction " developed in the mesophyll. 



5. The rarefaction of tracheal air must be attributed to rapid 

 removal of water from the vessels. 



6. The vessels and tracheides serve as water-conducting tubes, and 

 the water travels in their cavities. 



So far as small herbaceous plants are concerned, these facts are 

 quite sufficient to account for the upward movement of water in the 

 vessels and tracheides of the stem. In such plants, root-pressure is 

 well able to keep the entire tracheal system filled with water from the 

 roots right up to the tips of the leaves ; frequently, indeed, this pressure 

 is so great that special organs the hydathodes are brought into play 

 in order to get rid of the excess of water, and so to prevent the venti- 

 lating spaces from becoming flooded. On the other hand, the entire 

 conducting region lies within the sphere of influence of the osmotic 

 suction developed in the transpiring mesophyll. 



Very different conditions, however, prevail in the case of tall trees, 

 which naturally form the centra of interest in every enquiry into the 

 causes of the " ascent of sap." In these plants the vessels and 

 tracheides of the woody cylinder rarely contain continuous water- 

 columns during any part of the vegetative season ; the physiologist is 

 thus at once confronted with a host of difficulties, if he attempts to 

 discover the details of the mechanism underlying the ascent of sap 

 in tall trees, or even tries to determine what forces are available 



