258 PHYSIOLOGY OF TREES. 



any difference in the structure of the stems of 

 the Helianthus annua and the Helianthus tube- 

 rosus? In the latter, descending vessels must, 

 as it is said, exist; but in the former they must 

 be wanting. The petioles of all biennial plants 

 must be furnished with descending ducts tx) carry 

 the prepared sap to the roots or bulbs, in the Jirst 

 year, but must be destitute of such organs in 

 the second I Nay more ; if, as has been already 

 observed, a single leaf of a carrot or turnip, for 

 instance, stands over the winter, its function will 

 be different in the second than it was in the first 

 season. 



The descent of the sap is, however, a very old 

 idea, and has long been received as an esta- 

 blished fact. The following circumstances have 

 always been considered as proofs that there is a 

 downward as well as an upward course : viz. a 

 common holly or jasmine, budded with a varie- 

 gated sort, will afterwards throw up from the roots 

 variegated suckers. The attachment formed by 

 inserted buds or grafts and the stock is a union 

 of the cambium or vital membranes of both 



