VASCULAR BUNDLE SKELETONS. 



129 



in transverse section. In general, they are just to be distinctly perceived with the 

 unaided eye on transverse and longitudinal sections of the organs, especially by 

 transmitted light. When they contain strongly lignified elements, which of course 



FIG. zy^.—Satiibiicus Efiit/ns : the leaf-traces in two internodes. They 

 are arranged in a cylinder, which is here flattened out in one plane. Each 

 internode bears two opposite leaves, and each leaf receives from the stem 

 a median bundle {/;), and two strong lateral bundles {s's'). The descending 

 bundles split below, and the limbs so formed are inserted into the spaces 

 between the lower bundles. In addition to these, finer bundles {s" s") 

 exist, which are connected by horizontal branches from which bundles {nn) 

 ascend into the stipules. (After Hanstein.) 



FIG. iTß.—Asf'ülium fllix-ma. 

 vascular bundles (j.>) in the stem, 

 with the basal portions of finer bu 

 into the leaves. 



. h network of 

 l^ a single mesh, 

 idles which pass 



only occurs in land-plants, and when, in consequence of this, they are tough and 

 hard, they may often be drawn out from the tissues to considerable distances, in the 

 form of extensible elastic threads (leaf-stalks of Plantago major, Pri?mila sinensis) ; or 

 they may be laid bare in 

 large pieces by scraping off 

 the soft fundamental tissue, 

 as in numerous Ferns, e.g. 

 the stem of the Bracken, 

 Pkri's aqm'h'na, and in our 

 common male fern {Aspi- 

 diwn filix mas). After 

 taking away all the leaf- 

 stalks, it is possible by 

 means of careful pressure, 

 and bruising and washing, to 

 obtain even the whole vas- 

 cular bundle system of the 

 stem, as a hollow cylindrical 

 net-work. Particularly fine 

 objects, very instructive for 

 physiological purposes, are 

 obtained as so-called vas- 

 cular bundle skeletons, when 



suitable organs (parts of stems, foliage-leaves, fruits, &c.) are exposed to slow 

 rotting under water, by which all the softer tissue is destroyed. Fine vascular 

 bundle skeletons are often j)roduced spontaneously in the open by frequent 

 [3] 



FIG. \2A—Pteris aquilina. A transverse section of the subterranean stem (natural 

 size) : r hard, brown sub-epidermal tissue ; p soft, slimy parenchyma, rich in starch ; 

 pr dark walled sclerenchyma forming two broad bands traversing the stem ; ag fibre- 

 vascular bundles which run externally to these bands of sclerenchyma ; is bundles 

 running internally to the same bands— 5 the fibro-vascular bundle ag (in A) isolated by 

 scraping off the parenchyma ; it divides and branches. The dotted lines (k) show the 

 outline of the stem (st) and its branches (si' and sf) and a leaf-stalk [}>). 



