06 ORGANOGRAPHY. 



termed Acrog^jious (from two G-reek words signifying summit 

 ' growers), because such a stem 

 grows only by additions to its 

 summit. 



According to the views first 

 propounded by Schleiden, the 

 differences thus found to exist 

 in the appearance and growth 

 of these three kinds of stem 

 are due to corresponding dif- 

 ferences in their fibro-vascular 

 systems, or as they are com- 

 monly called, fibro-vascular or 

 vascular bundles. Thus the 

 vascular bundle of an Exo- 

 genous stem {fig. 1 58) consists 

 in the first year of growth of 

 JV^. 157. Transverse section of the stem of a layer of spiral vessels, S V, 

 a Tree-fern. m. Parenchymatous cells, surrounding the pith 'p; on the 

 which are wanting in the centre. '»,v,v. .-^ r- ^i^- ]„„„_ thpro 



Fibro-vascular bundles, e. Rind. outsiae 01 tnis layer mere 



are subsequently developed in 

 perennial plants, pitted vessels and wood-cells, which together 

 form the wood, w. In herbaceous plants, annular and reticulated 

 (Vessels are also found with the wood-cells. In this case the grovrth 

 /of the difi^erent parts of the bundle is progressive, 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 layer 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 succession on the outside, and in continuity 

 with the previous ones, as long as life continues, Henca such are 

 called indefinite or continuous vasndar bundles. 



In Endogenous stems the vascular bundles {fig. 159) consist 

 internally of wood-cells, w, and spiral vessels, s v; on the outside 

 of which other spiral vessels are formed, as well as pitted, d, and 

 other vessels ; these are bound together and succeeded by a number 

 of delicate parenchymatous cells, d, and on the outside of these 

 some laticiferous vessels, / c, and at first a cambium region, c, 

 which is gradually converted into thick-sided prosenchymatous 

 cells, I, resembling those of the liber of Exogens. In this case 

 the development of the vascular bundles, like those of Exogenous 

 stems, is gradual, the inner part of each being first formed, and 

 growth proceeds progressively to the outside ; hence such are like- 

 wise progressive bundles, but, as such bundles have no external 

 layer of growing cells resembling the cambium layer, no increase 

 in size takes place in them in successive seasons. Hence the new 

 vascular bundles are not developed in continuity with the old, 



