204 PHYSIOLOGICAL BOTANY. PART II. 



are subjected. The first of these is its descent and 

 transfusion ; the second is a very singular rotation 

 of the juices contained in the vesicles and short tubes 

 of some plants ; and the third is a sort of actual 

 though local circulation more nearly resembling the 

 circulation of blood in animals. We propose to describe 

 each of these under the present period, though certainly 

 they can hardly be all considered as subordinate pro- 

 cesses of the same function. 



(190.) Descent of Sap. When a ring of bark is re- 

 moved from a stem or branch of a dicotyledonous plant 

 a tumour is formed at the upper edge of the ring, which 

 indicates a stoppage to have taken place in the descent 

 of the elaborated sap. This stoppage by causing an 

 excess of nutriment to accumulate above the ring, oper- 

 ates in improving the size and quality of fruits, and 

 will even occasion a tree to flower and produce fruit 

 when it would otherwise have developed nothing but 

 leaves. No increase or at most a very slight one takes 

 place in the diameter of the trunk below the ring; but the 

 part above it is more developed than it otherwise would 

 have been. If a potato be ringed in this way the buds 

 in the axillte of its leaves are developed in the form 

 of little tubers, whilst none are produced on the under- 

 ground stems or rhizomata. Similar effects are produced 

 by a tight ligature; and most persons have observed the 

 appearance which a woodbine causes on the branches of 

 trees by twining round them. A spiral protuberance 

 is formed immediately above and below the stricture, 

 but more especially above it, and in process of time 

 these swellings often become so large as to meet com- 

 pletely over the woodbine and embed it in the sub- 

 stance of the tree. The parts which lie above a ring 

 or ligature become specifically heavier than those which 

 are below it as Mr. Knight found in the oak, the 

 wood above having a specific gravity of 1'14, and that 

 below only I'll. All these facts seem to indicate 

 that the chief passage of the descending sap is down 

 the bark, and towards the surface of the stem. It was 



