12 RISE OF SAP 



hindering the ready acceptance of the doctrine, that the absorption of 

 moisture by the spongioles of a tree is the homologue of the absorp- 

 tion of moisture by the whole surface of the confervse of the streamlet 

 and of the algse of the sea, will be removed. , 



I have oftener than once made use of the term absorption, but 

 as may have been seen, from the explanation of exosmose and endos- 

 mose which has been given, it is scarcely apphcable with propriety to 

 such a process ; but with this caveat I may be allowed to make use 

 of it in lack of some term less exceptionable. It is solely in reference 

 to that process that I here employ it. 



The water thus absorbed by the several cells composing the 

 spongioles of the root is by a similar process absorbed from them by 

 cells behind them ; and by continuous repetition of it by those beyond 

 the moisture absorbed from the soil is passed on and on, from the 

 extremities of the rootlets to the extremities of the smallest twigs, 

 and to the furthest and the loftiest extremities of the branches of the 

 trunk. There, through the leaves, a part, and that a large portion 

 of it, is given off into the atmosphere, while a part, comparatively a 

 small portion, is returned by the same duplex process of exosmose and 

 endosmose by the same cells, and others, their progeny, towards the 

 root. By the way is deposited, by exosmose, nutriment for the tree, 

 the leaf, the flower, and the fruit; and the residuum is in part deposited 

 by the same process in the leaves, the bark, or the root, and passed 

 off into the soil by the exosmosic action of the cells composing the 

 spongioles of the root. 



It may now be seen how vegetation may have some effect in 

 increasing the humidity of the atmosphere ; but, as a preliminary to 

 the consideration of this, some further attention may be given to the 

 phenomenon of the rising of the sap. 



The rise of the sap in trees is to many a phenomenon or fact for 

 which they cannot account satisfactorily to themselves ; or, if this 

 they can do, or think they can, they cannot do so in such a way as to 

 cover all the phenomena to the satisfaction of others. The pressure 

 of the atmosphere will not account for what is seen. Capillary 

 attraction has been thought of, but this does not account for a down- 

 ward flow of the sap, which is a part of the phenomenon, and it is a 

 downward as well as an upward flow which has to be accounted for. 

 Experiments have been made with a view to account for the upward 

 flow, the results of which, and perhaps some others, could be 



