58 COMPOUND ORGANS OF PLANTS. 



tubes of woody' fibre, c c, intermixed with bothrenchyma or 

 porous vessels, h b. On holding up a thin traverse section of 

 an oak or ash stem to the light, the porous vessels will be seen 

 in the form of large round openings in the tissue, which, 

 situated near the margin of each woody circle or zone, renders 

 apparent the annual growths of the stem. In the maple, 

 plane, and lime tree, these openings are smaller and more dif- 

 fused, and hence there is an indistinctness in the line of demar- 

 cation between the successive zones. 



This new layer of wood gradually loses its softness as the 

 sea,son advances, and towards the middle of winter is condensed 

 into a solid ring of wood. In this country, and in Europe 

 generally, there is a periodical check to vegetation during the 

 colder part of the year, which occasions the annual layers 

 found in the stem of exogenous trees. These annual rings, 

 which are distinctly seen in most trees of temperate climates 

 when a section of their stem is examined, serve as natural 

 marks by which to distinguish their age. Thus, suppose an 

 elm, or any other tree to be felled, and the section near the 

 ground to have thirty-five circles, or rings of wood, it may be 

 inferred that the tree is thirty-five years of age. 



This computation, however, can only be made in trees which 

 have these rings distinctly marked, and even then there are 

 sources of deception of which it is proper that the student 

 should be informed. For example, a warm spring followed by 

 weather cold enough to check vegetation, will leave a ring in 

 the stem, and the subsequent growth of the stem, with the 

 return of warm weather will give on the cross-section the 

 appearance of two rings of wood, or of two years growth, to the 

 growth of one year; on the other hand a warm winter, by 

 keeping the tree constantly growing without check, will give 



