28 STEM 



cotyledon cannot increase in thickness when it has one? 

 ceased to elongate. But the reverse is the case with Dicoty- 

 ledonous trees. An old part of an Oak stem is much 

 thicker , than a young part near the apex : the older the 

 stem the thicker it is. After the stem of a Dicotyledon has 

 ceased to elongate it may be able to grow in thickness. If we 

 examine the trunk of an oak-tree which has been cross-cut, 

 we note the bark lying outside the wood. In the centre of the 

 trunk there is a dark patch of heart-wood, which is surrounded 



44 45 46 



Figs. 44-47. — Cross-sections of steins showing annual thickening rings of wood. 

 Fig. 44 is one year old. Fig. 45 is two years old. Fig. 46 is three years old. Fig. 47 

 is five years old, and shows the bark peeling off. r denotes wood formed in the first 

 year ; 2, the wopd formed in the second year ; and so on. 



by the lighter-coloured splint-wood. Still looking at the cross- 

 section of the trunk, we note that there are a number of ring-like 

 markings ranged round the centre. We find that in a two-year- 

 old stem there are two rings, in a three-year-old stem three 

 rings ; in fact, that the number of rings corresponds with the 

 number of years of growth of that portion of the stem. For this 

 reason the rings are termed annual rings. Each ring denotes one 

 year's growth in the thickness of the stem. There are also 

 numberless radial lines which are the medullary rays. The 

 knots met with oh cutting across timber are the remains of 

 portions of branches, which have been buried in the wood as 

 the stem thickened. 



SUBTERRANEAN SHOOTS. 



A stem may protrude into the air and be sub-aerial; or live 

 under water and be submerged; or lie buried in the soil and 

 be subterranean. 



In the case of many plants only a part of the shoot is raised 



