110 



BIOLOGY 



and the bark also increases in thickness by growing on its 

 inner side. This growth is, however, not so vigorous as is that 

 of the wood, and the bark does not increase in thickness so 

 much as does the stem. Since too the new cells of the bark 

 are deposited on the inner side, the older parts of the bark 

 must stretch to cover the increasing diameter of the growing 

 stem. When a stem becomes of considerable size the outer 

 bark will be found to be rough and broken by the expansion 

 of the stem which it covers. 



Some plants, which have but one year's growth, form a 

 single ring of wood as described, and die at the close of the 

 season. Other plants, like large trees, do not die, but live 

 year after year; and each year the cambium layer adds new 

 masses of cells outside of those previously existing. In plants 

 that live in regions where the climate changes with the seasons, 



the cells formed by the 

 cambium layer are larger 

 at certain seasons of the 

 year than at others. In 

 temperate regions, the 

 wood cells formed in the 

 spring are larger and 

 relatively thinner walled 

 than those formed later 

 in the season. During 

 the winter, growth ceases 

 entirely; but as soon as 



-C 



FIG. 50. SECTION ACROSS AN EXOGENOUS 



STEM OF FOUR YEARS* GROWTH, SHOW- 

 ING THE FOUR RINGS OF WOOD 



spring comes again, a 



new layer of large cells 

 will be deposited on the 

 outside of the last ring 

 that was deposited in 

 the fall. The result of 



this is a series of rings easily recognized when a cross section 

 of a stem is made; Fig. 50. Since each ring indicates ordi- 



6, bark; 



c, cambium layer; 



w, wood ring. 



