64 PLANT BIOLOGY 



as large at the top as at the base. As dicotyledonous 

 plants grow, the stems become thicker each year, for the 

 delicate active cambium layer forms new cells from early 

 spring until midsummer or autumn, adding to the wood 

 within and to the bark without. As the growth in spring 

 is very rapid, the first wood-cells formed are much larger 

 than the last wood-cells formed by the slow growth of the 



FIG. 77. WHITE PINE STEM, 5 years old. The outermost layer is bark. 



late season, and the spring wood is less dense and lighter 

 colored than the summer wood ; hence the time between 

 two years' growth is readily made out (Figs. 77 and 78). 

 Because of the rapid growth of the cambium in spring and 

 its consequent soft walls and fluid contents, the bark of 

 trees " peels " readily at that season. 



Medullary Rays. The first year's growth in dicotyle- 

 dons forms a woody ring which almost incloses the pith, 

 and this is left as a small cylinder which does not grow 



