TREE STEMS. 



669 



surrounds a half-moon-shaped bundle of bast-cells, c, which are separated 



in the direction towards the interior, by a layer of cambium, dd'd", from 



the bundles of vascular tissue, consisting of 



vessels and longitudinal cells. The latter tissues 



may be distinguished in the transverse section 



by the thicker walls, gg % and by their greater 



breadth, hh. It is further to be remarked that 



the cambium transparent tissue, d d", appears on 



both sides of the bundles of vascular tissue, and 



extends to the next bundle, and thus presents 



an uninterrupted circle throughout the entire 



circumference of the stem. 



On examining the sec- 

 tion Of a One-year-Old Fig. 75 7 .-Dicotyledonous stem. 



dicotyledonous stem, magnified six times, as in 

 fig. 7 5 8, we perceive several parts clearly distinguish- 

 able from each other, corresponding with the ar- 

 rangement of the bundles of vascular tissue. 



Enclosed by the epidermis, a, is a large-celled 

 tissue, bf and ;;/, in which a number of vascular 

 bundles form a circle. In each of these we notice 

 that the outer portion, consisting of bast-shell, r, is separated by the cambium, 

 d, from the inner woody portion, e. The cambium forms a closed circle which 

 penetrates through all the vascular bundles. 



In the course of the further development of the stem, the parts, a b c, 

 constitute the bark, the vascular bundles, e> the wood, and the cellular tissue, 

 /, its pith. The tissues, m, penetrating between the vascular bundles, are 

 called the medullary rays. The cambium is to be regarded as the most 

 important part, since it' is the source of new bundles of vascular tissue which 

 year by year increase the circumference of the stem. 



Fig. 758. Stem one year old. 



Fig. 759. Stems three and five years old. 



The growth of a dicotyledonous stem is continued by the formation of 

 a new circle of vascular bundles on the circumference of the stem in the 

 second year. Each new bundle, as has already been shown, is produced 

 in the cambium, and consequently is deposited between the wood and inner 

 bark. 



Thus every year a new layer is deposited between the previous forma- 

 tion and the bark ; and a section will exhibit these concentric rings of wood 



