ORGANIZATION OP THE STEM. 59 



the appearance of one layer to the growth of two years. Not- 

 withstanding this, practical men find counting the concentric 

 circles of exogenous stems, to be the best mode which has yet 

 been discovered for ascertaining their age, as in ordinary cases 

 only one growth is made in the course of a year. 



In tropical countries, where the temperature is compara- 

 tively speaking pretty much the same throughout the year, 

 these rings are very indistinctly marked. In tropical coun- 

 tries, vegetation is not liable to that periodical check which it 

 receives in colder regions, and therefore the cross-section of 

 the stems of exogenous trees in many instances do not disclose 

 these rings, or any separation of the wood into concentric 

 layers. 



As the same development of woody and cortical matter takes 

 place in the branches as well as in the stems of exogenous 

 trees, therefore, the time when a branch was first given off 

 from the stem may be computed by counting the circles on the 

 stem and branch respectively. If there are, for instance, 

 thirty rings in the stem, twenty in one of its branches, and 

 five visible on the cross-section of another, then the tree must 

 have been ten years old when the first branch was developed, 

 and twenty-five years of age when it formed the second. 



If we carefully examine the rings on the cross-section of an 

 exogenous stem, we shall soon perceive that they have not the 

 same geometrical centre, that their breadth varies, and that 

 they are occasionally thicker on one side of the tree than on 

 the other. A variety of causes contribute to produce these 

 effects. The variable breadth of the rings depends on the 

 variability of the seasons ; for more wood will necessarily be 

 deposited when the season is favorable for vegetable growth, 

 than when the contrary is the case. Moreover the circles will 



