WOOD AND BARK. 



179 



tissue, more abundantly on its inner side for wood 

 than on its outer side for bast, and the former has 

 hardened year by year into the four rings of wood. 

 This is how it is that in the trunks of many trees you 

 can often plainly see the rings of wood as many 

 rings as there are years in the age of the tree * (cf. 

 Figs. 157, 155). 



In Fig. 1 5 8 you can see 

 that there are slits or 

 openings between the 

 fibro-vascular bundles, 

 which go from the cam- 

 bium towards the pith 

 or medulla. These are 

 called medullary rays, 

 and are filled with fun- 



i . / Fig. 158. Portion of a stem when cut 



damental tissue (paren- across, showing surface (transverse 



rhvma"! Tripv nfr-p-n section ) mi '> and surface whe.n cut 

 1 lengthwise (longitudinal section) a to 

 add greatly to the h lhe P ith ? m shows medullary rays. 



beauty of the wood, as for instance in what is known 

 as silver grain. In a transverse section of a stem, 

 these medullary rays appear as rays from the pith 

 towards the bark (Fig. 157). And compare Figs. 152 



* Because the new wood is added to the outside of that already 

 formed, stems of this kind of growth (dicotyledons, p. 176) have been 

 called "exogenous" The word is derived from two Greek words 

 "^-," "exo" outwards, on the outside, and " genu" old form of 

 " gignomai" to come into being. The stem increases in thickness by- 

 addition upon the outside of the wood only. When the fibre-vascular 

 bundles grow here and there amongst the fundamental tissut 

 irregularly, the plants have been called '''endogenous'" (" endonj 

 within, and "geno"}. If you cut across a cane, you can see a good 

 example of this growth ; and see Fig. 154. 



