VASCULAR BUNDLES. [ 801 ] VASCULAR BUNDLES. 



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 

 in the Ferns. In the Ferns, the ducts are 

 mostly of the kind called scalariform (fig. 

 664, page 712 j PI. 48. fig. 10), in the Equi- 

 setacec'e annular (fig. 661, page 711), in the 

 Lycopodiacese spiral (fig. 659, page 711 ; 

 PI. 48. figs. 11 & 12). They are variously 

 arranged in the different orders, but agree in 

 the mode of development, namely in grow- 

 ing only at the end next the punctum vege- 

 tationi*, in proportion to the elongation of 

 the stem and the evolution of leaves. Hence 

 Schleiden called them simultaneous bundles ; 

 their various elements ducts, tubular and 

 prosenchymatous cells being formed si- 

 multaneously. 



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. 461, page 508). Here 

 (fig. 791) the first trace of the vascular bundle 

 consists of spiral vessels, followed on the 

 outer side by spiral, annular, or reticulated 

 ducts 5 next comes a collection of elongated 

 tubular cells of delicate structure (vasa 

 proprid), and in the outer part, at first, a 

 cambium region, which is gradually con- 

 verted into prosenchymatous woody struc- 

 ture having the character of LiBER-cells. 

 In this case, the development is not only 

 gradual from the punctum vegetationis out- 

 ward, 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 

 progressive bundles ; but as no new develop- 

 ment occurs in these in successive seasons, 

 they are further distinguished from those of 

 the Dicotyledons as definite bundles. The 

 structure of the vascular bundles of Monoco- 

 tyledons is very well seen, indifferent charac- 

 teristic conditions, in vertical and horizontal 

 sections of the stems of the white lily, of the 

 large grasses,rhizomes of sedges and rushes 

 affording well-developed examples in herba- 

 ceous structures ; of the bamboo (an arbo- 

 rescent grass), of the common cane or the 

 partridge cane (both species of Palms), 

 where the bundles are connected by lignified 

 parenchyma* In leaves of bulbous Monoco- 



tyledons, &c., the bundles consist chiefly of 

 spiral vessels ; in the palms, bananas, &c., 

 the woody fibre extends also into the ribs of 

 the foliaceous organs. 



In the Dicotyledons, the bundles of the 

 stem appear first as a circle of cords com- 

 posed or spiral vessels, around the pith, out- 

 side whicn larger vessels and ducts, and 

 subsequently woody fibre or wood-cells are 

 developed, passing into the elongated pros- 

 enchymatous liber (fig. 792). The develop- 



' Fig. 792. 



Dicotyledon. 



Transverse section of a flbro-vascular bundle of a 

 Melon stem ; the upper end next the centre of the stem. 

 p, pith ; s. v, spiral vessels ; TO. r, medullary rays ; w, 

 wood ; d, pitted ducts ; c, cambium ; I, liber ; I. c, lati- 

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

 epidermis. Magnified 50 diameters. 



ment of the successive regions is progressive 

 during the first season ; but here the cam- 

 bium layer remains capable of renewed 

 activity, and a new layer of wood (and of 

 liber) is added on the outside of the bundle 

 in each successive season j hence these bun- 

 dles are distinguished as indefinite. These 

 may be observed in sections and young 

 shoots of any common tree (figs. 455 & 457, 

 pages 495 & 496). 



Infinite variety of modification occurs in 

 the character and arrangement of the vas- 

 cular bundles within the limits above laid 

 down, or very slightly overstepping them. 

 A few remarkable cases may be mentioned 

 here ; in the Orobanchaceae (parasites) no 

 spiral vessels occur in the vascular bundles 

 forming the wood; in Victoria regia the 

 isolated bundles are composed of spiral 

 vessels without any prosenchymatous wood- 

 cells j other peculiarities, influencing more 



