66 



STRUCTURE OF THE STEM 



With the increase of the fibro-vascular bundles of the wood, 

 the space between them, at first large, becomes less, and the 

 pith, which extended freely out toward the bark, becomes com- 

 pressed into thin plates so as to form medullary rays. These 

 are, as the starch test shows, useful in storing food which 

 the plant in cold and temperate climates lays up in the sum- 

 mer and fall for use in the 

 following spring, and in 

 the very young stem they 

 serve as an important 

 channel for the transfer- 

 ence of fluids across the 

 stem from 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 interspaces be- 

 tween the first fibro-vascu- 

 lar bundles have become 



FIG. 67. Cross section of a three-year-old 

 linden twig 



e, epidermis and corky layer of the bark ; 

 b, bast ; c, cambium layer ; r, annual 

 rings of wood. Much magnified. After 

 Kny 



filled up with wood, the subsequent growth must take place in 

 the manner shown in Fig. 68. The cambium of the original 

 wedges of wood /c, and the cambium ic formed between these 

 wedges, continues to grow from the inner and from the outer 

 surface, and thus causes a permanent increase in the diameter 

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

 ally soon begins to peel off from the outside and thus remains 

 pretty constant in thickness. 



