xiv 



INTRODUCTION. 



wliich substance pith was formerly supposed to be 

 analogous). The medullary sheath retains its 

 vitality after the pith itself has ceased to per- 

 form its functions, and extends its spiral vessels 

 into the leaves, flowers, and fruit. Immediately 

 without this is a dark-coloured cylinder, consist- 

 ing of woody-fibre and ducts cemented together, 

 as it were, by cellular tissue. This layer was de- 

 posited during the first year in wliich the tree 

 reached the height at which the section was 

 made ; it was then white, and is supposed to have 

 derived its dark colour from the proper juices dis- 

 charged by the ducts. The term heart-wood is 

 employed to distinguish it after it has acquired 

 this colour. The first layer of heart-w^ood is en- 

 closed in a second, precisely similar, except that 

 the medullary sheath is absent ; and the whole 

 substance of the trunk is made up of layers of 

 the same form and structure, the exterior circles 

 being softer and colourless, whence the outer 

 wood is called alburnum (from alhus, w^hite). 

 Each of these layers having been deposited during 

 the growing season of a single year, there will be 

 little difficulty in fixing the age of the tree, pro- 

 vided that it has not been checked by transplan- 

 tation, and that it has been subjected to a climate 

 where the termination of one year's growth and 

 the beginning of the next have been well defined 

 by the intervention of winter, as a period of rest. 

 In hot climates trees experience no season of 

 perfect cessation from growth ; and when this is 

 the case, the annual layers are so confused, as to 

 permit no certain criterion of age. The same 

 may be said of trees of great antiquity, for in 

 these also the aimual circles are often imperfectly 



