VASCULAR BUNDLES. [ 669 ] VASCULAR BUNDLES. 



tyledons, or are only combined into a wood, 

 in the Palms, &c., by the lignification of the 

 cells of the parenchyma in which they are 

 imbedded (fig. 465, p. 430). In the Dicoty- 

 ledons, the rudimentary bundles are deve- 

 loped in a circle surrounding the pith (tig. 

 459, p. 419), and soon unite to form a tube 

 of wood, with an external cambium layer and 

 a true bark, and the cambium layer is the 

 seat of renewed development of the vascular 

 bundle in each successive year. On such 

 characters of growth, Schleiden founded a 

 division of the vascular bundles into classes, 

 which are convenient in reference to micro- 

 scopical investigations, and affixed tolerably 

 perfect systematic characters for the classes. 

 In the higher Flowerless Plants, viz. 

 Ferns, Equisetacese, &c., the vascular bun- 

 dles are composed chiefly of ducts, sur- 

 rounded by elongated tubular cells, almost 

 devoid of secondary deposits, the whole 

 enclosed by a layer of tolerably firm prosen- 

 chymatous wood-cells, especially developed 



Fig. 796. 



l.f 



Monocotyledon. 



Transverse section of a fibro- vascular bundle of a Palm; 

 the upper end is directed towards the centre of the stem. 

 w, woody fibres resembling liber in structure ; s. v, spiral 

 vessels ; c, cambium (vasa propria} ; d, ducts ; p, paren- 

 chyma ; /, liber ; /. c, laticiferous canals. Magnified 150 

 diameters. 



in the Ferns. In the Ferns, the ducts are 

 mostly of the kind called scalariform (fig. 

 669, page 593 ; PL 39. fig. 10), in the Equi- 

 setacea3 annular (fig, 666, page 593), in the 

 Lyeopodiacese spiral (fig. 664, page 592; 



PI. 39. figs. 11 & 12). They are variously 

 arranged in the different orders, but agree in 

 the mode of development, namely in grow- 



Fig. 797. 





Dicotyledon. 



Transverse section of a fibro-Tascular bundle of a Melon 

 stem ; the upper end next the centre of the stem, p, 

 pith ; s. v, spiral vessels ; m. r, medullary rays ; w, wood ; 

 d, pitted ducts ; c, cambium ; /. liber ; /. c, laticiferous 

 canals ; c. e, cellular envelope of the bark ; e, epidermis. 

 Magnified 50 diameters. 



ing only at the end next the punctum vege- 

 tationis, in proportion to the elongation of 

 the stem and the evolution of leaves. Hence 

 Schleiden calls them simultaneous bundles, 

 their various elements, ducts, tubular and 

 prosenchymatous cells, being formed simulta- 

 neously. 



In the Monocotyledons, where the vas- 

 cular bundles occur isolated, they originate 

 in the punctum vegetationis, and are deve- 

 loped with the growth of the stem outwards 

 and upwards into the leaves, and outwards 

 and downwards towards the permanent cir- 

 cumference of the stem, old and new bundles 

 crossing each other in a more or less com- 

 plicated manner (fig. 465, page 430). Here 

 (fig. 796) the first trace of the vascular bundle 

 consists of spiral vessels, followed on the 

 outer side by spiral, annular or reticulated 

 ducts ; next comes a collection of elongated 

 tubular cells, of delicate structure and in the 

 outer part, at first a cambium region, which 

 is gradually converted into prosenchymatous 

 woody structure, having the character of 

 LiBER-cells. In this case, the development 

 is not only gradual from the punctum vege- 

 tationis outward, but the inner side of each 

 bundle is perfected first, and the conversion 

 of the outer part into wood occupies a whole 

 season of growth. Hence these are entitled 



