OF VITAL MOTION. 11 



Section I. 



OF VITAL MOVEMENTS IN THE VESSELS, CELLS, AND 

 INTERCELLULAR PASSAGES OF THE PLANT. 



Preliminary Considerations. 



In plants, the sap exhibits very positive motion, but 

 there is no true circulation. There is a general move- 

 ment which belongs to the plant as a whole, and to 

 which all parts contribute; and there is a special 

 movement, which is itself divided into two varieties, 

 the one belonging to the laticiferous vessels, and the 

 other to certain single cells. 



The plan of the general movements of the sap would 

 seem to vary at different times and places. When 

 examined during the summer, the main current is 

 found to pass from the roots, through the fibres and 

 ducts of the newly-formed woody zones of the stem, to 

 the upper surface of the leaves. It then tends towards 

 the lower surface of the leaf, and thence, in a down- 

 ward direction, through the fibrous structures of the 

 bark, passing at the same time inwardly and horizon- 

 tally through the tissues which form the medullary 

 rays. " Very little of the elaborated sap reaches the 

 roots, from which the motion commenced ; and none of 

 it, except that small quantity which mixes with the 

 ascending current, is again transmitted through the 

 system." The movement is not confined to any special 

 tissue, but it seems to pervade indiscriminately the 



