THE STEM 47 



more than that of the bark. This is well seen by ex- 

 amining the cut end of a felled tree, for instance, an elm 

 or oak ; the enormous difference in thickness between 

 such an old tree and a seedling elm or oak being due 

 almost entirely to the additions made to the wood by 

 the activity of the cambium layer. The presence of a 

 cambium is practically restricted to dicotyledonous 

 plants. 



Certain changes take place in the wood of many 

 trees as it increases in age. From what has already 

 been said, it will be recognised that the oldest wood is 

 near the centre, the new wood being formed always on 

 the outside. It is not uncommon to find the wood near 

 the centre of the trunk darker in colour. This is par- 

 ticularly well seen in the laburnum, where the centre 

 part is deep brown and the outer portion light yellow. 

 The elm, oak, etc., show the same, although to a less 

 striking degree. This central dark wood is the heart- 

 wood, and the outer softer and lighter-coloured wood 

 the sapwood. 



The rate of formation and the character of the new 

 wood formed from the cambium varies at different 

 season's 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, and by counting the rings the age 

 of the tree can be told. In tropical countries the seasons 

 are often not sharply marked off, and the rings of growth 

 are accordingly often wanting or indistinct. 



Close examination of a cross-section of a stem reveals 



