72 



ORGANOGRAPHY. 



differences thus found to exist in the appearance and growth of 

 these three kinds of stem are due to corresponding differences 

 in their fibro-vascular systems, or as they are commonly called, 

 fibro-vascular or vascular bundles. Thus the vascular bundle of 

 an Exogenous stem {fig. 166) consists in the first year of growth 

 of a layer of spiral vessels, sv, surrounding the pith, p; on 

 the outside of this layer there are subsequently developed pitted 

 and other vessels, and wood-cells, which together form the 

 wood, w. In this case the growth of the different ])arts of 

 the bundle is progi-essive, and the whole is covered externally 

 by a layer of vitally active cells called the cambium layer, c, on 

 the outside of which is the liber, I, and the other parts of the bark, 

 c e. It is from the cambium that new layers of wood are formed, 

 and from its position therefore on the outside of the vascular 

 bundles, their growth is indefinite, as they are deposited in succes- 

 sion on the outside, and in continuity with the previous ones, as 

 long as life continues. Hence such are called indefinite vas- 

 cular bundfes. 



Fig. 166. 



Fig. 16; 



Fig- IW. Transverse section of a fibro-vascular bundle of an Exopenous 

 stem (Melon). 7). Pith. x?-. Spiral vessels, hi r. Medullary rays. «•. Wood. 

 (/. Pitted vessels, r. Cambium. /. Liber. 1c. Laticiferous vessels, ft'. 



Cellular portion of the bark. 1. Epidermal tissue Fig. 1(>7. Transverse 



kection of a fibro-vajiulor bundle of an Endogenous stem (Palm), the 

 upper portion being directed to the centre, w. Wood-cells. in\ Spiral 

 vessels, c. Cambium. </. Pitted vessels, p. Farencliyma. /. Liber-cells. 

 Ic. Laticiferous vessels. 



In Ktidogenous stems the vascular bundles {fig. 167) consist 

 internally of wood-cells, w, and spiral vessels, sv \ on the outside 



