STRUCTURE OF STEMS. 47 



is between wood and bark, it can add both to 

 the wood and to the inner bark. The increase 

 in thickness of the wood is generally very much 

 more than that of the bark. This is well seen 

 by examining the cut end of a felled tree for 

 instance, a mango ; the enormous difference in 

 thickness between such an old tree and a 

 seedling mango being due almost entirely to 

 the additions made to the wood by the activity 

 of the cambium layer. The presence of a cam- 

 bium is practically restricted to dicotyledo- 

 nous plants. 



"3. The rate of formation and the charac- 

 ter of the new wood formed from the cambium 

 varies at different seasons of the year. Thus, 

 when a cross-section of a stem is looked at, 

 rings or layers in the wood are visible. Trees 

 grown in countries having well marked seasons 

 of winter and summer, usually show a definite 

 ring for each year's growth. In the West 

 Indies as in other tropical countries the 

 trees generally show only irregular rings, being 

 determined by less regular changes such as 

 the alternation of wet and dry periods. 



4. Close examination of a cross-section 

 of a stem reveals the presence of fine lines- 

 well seen in a rose stem running through 

 the wood joining up pith and cambium. These 



