STRUCTURE OF THE STEM 



73 



bark to pith, or in, the reverse direction. On account, 

 perhaps, of their importance to the plants, the cells of 

 the medullary rays are among the longest lived of all 

 vegetable cells, retaining their vitality in the beech tree 

 sometimes, it is said, for more than a hundred years. 

 After the inter- .T^-vrra™-™™. 





spaces between the 

 first fibro- vascular 

 bundles have be- 

 come filled up with 

 wood, the subse- 

 quent growth must 

 take place in the 

 manner shown in 

 Fig. 44. The cam- 

 bimn of the original 

 wedges of wood,/e, 

 and the cambium, ic, 

 formed between 

 these wedges, con- 

 tinues to grow from 

 its inner and from 

 its outer surface, 

 and thus causes a 

 permanent increase 

 in the diameter of 

 the stem and a thickening of the bark, which, however, 

 usually soon begins to peel off from the outside and thus 

 soon attains a pretty constant thickness. 



89. The Dicotyledonous Stem, thickened by Secondary Growth. — 

 Cut off, as smoothly as possible, a small branch of hickory and one 

 of "white oak above and below each of the rings of scars already 



Fig. 45. Cross-Section of a Three- Year-Old 

 Linden Twig. (Much magnified.) 



E, epidermis and corky layer of the bark ; B, 

 bast; C, cambium layer; E, annual rings 

 of wood. 



