336 CONSERVATIVE ORGANS. [LECT. VII. 



Wood in its soft state, or that in which it 

 forms the outer circle in every ligneous dicotyle- 

 donous stem and branch, is, as has been already 

 mentioned, named alburnum. While it con- 

 tinues so, it is endowed with nearly as much ir- 

 ritability as the liber; and, as shall be after- 

 wards fully described, performs functions of 

 great importance in the vegetable system ; but 

 when it becomes hard these functions cease, and 

 in time it loses even its vitality ; not unfrequently 

 decaying in the centre of the trunk of trees ; 

 which, nevertheless, still flourish and put forth 

 new shoots as if no such decay existed. To 

 carry on, therefore, the functions of the wood, a 

 new circle of it is annually formed over the 

 old ; and thus, also, the diameter of the trunk 

 and branches present, by the number of these an- 

 nual zones, a pretty correct register of their age, 

 each zone marking one year in the life of the 

 part*. There are, however, exceptions to the 

 criterion thus afforded of the age of the plant, for 

 circumstances may occur to prevent the zone from 

 being formed of a thickness which will be per- 

 ceptible after a few years have passed over, and it 

 is pressed between other zones. If the summer 



* Vide Plate 6, figures 3, 4, 5, which represent sections of 

 the same stem, in the first, second, and third year of its 

 growth ; and fig. 6. which displays two zones, as they are seen 

 in a longitudinal section of the stem of the Elder. 



