48 BOTANY. [chap. i. 



have increased to a yard in diameter, the latter may not 

 be half an inch thick. 



All European forest trees are Dicotyledons, and the 

 ringed appearance seen in a trunk that has been sawn 

 across is due to the periodical additions made by the 

 cambium to the xylem or wood. In temperate regions, 

 where there is only one season of growth during the 

 year, a single ring is formed annually, hence the term 

 annual rings, as applied to these markings, which may 

 be used in determining the age of the tree with approxi- 

 mate accuracy ; such rings give at least the minimum 

 age of the plant. In the tropics, where there is more 

 than one growing season during the year, two or more 

 rings are formed by some species in one year. 



The popular expression " rising of the sap " corre- 

 sponds to the renewed growth of the cambium in the 

 spring ; the tender mass of thin-walled tissue thus 

 formed readily admits of the removal of the outer 

 portion, or " bark." 



The distinct appearance of the annual rings of wood 

 originates as follows : — The first additions to the wood 

 made by the cambium during the spring, consists for 

 the most part of vessels of large diameter and with com- 

 paratively thin, pitted walls ; the formation of these 

 vessels is favoured in the spring by the slight amount of 

 pressure exerted by the bark, that has been kept moist 

 during the winter season, but as the summer advances 

 the bark becomes dry and rigid, and the pressure on 

 the newly-formed wood becomes greater, consequently 

 the formation of vessels in the xylem is superseded by 

 that of wood cells with very small cavities and thick 

 walls, the result being that the porous spring wood. 



