LECT. VII.] ANATOMY OP STEMS. 355 



takes place, and the new wood will always remain 

 distinct*. In this state, also, through its tubes 

 the sap is raised to the summit of the highest 

 trees ; and, according to Mr. Knight, when this 

 function ceases in winter, it becomes " a reservoir 

 " of the sap or blood of the tree, as the bulbs of 

 " the Hyacinth and Tulip, and the tuber of the 

 " Potatoe certainly do of the sap or blood of these 

 " plants ;" an analogy which is good ; but we must 

 remark, in making this admission, that it is not 

 the sap, but the proper juice (succus proprius), 

 which is there laid up, and is dissolved by the 

 first ascending sap in the spring. 



The alburnum does not attain its entire thick- 

 ness at once ; but continues to increase during 

 the greater part of the summer, at least till after 

 the midsummer shoots are protruded ; and even, 

 if circumstances occur to stop its progress, such 

 for example as a week of very cold weather 

 in the middle of the season, two layers may be 

 formed in the same year ; an event not unlikely to 

 happen in this variable climate, and which throws 

 an uncertainty on the mode, which has been al- 

 ready alluded to, of determining the age of trees 

 by counting the number of the concentric zones of 



* In this new wood the divergent rays are distinctly seen ; 

 a sufficient reason, as Mr. Knight remarks, for believing that 

 they are not processes of the pith. 



AA2 



