298 



THIRD GROUP.— VASCULAR CRYPTOGAMS. 



reticulated and spiral tracheides with thin-walled parenchymatous cells irregularly 

 distributed among them ; the vascular portion thus constituted is surrounded by a 

 mantle of tabular cells which have broad pits but no sieve-pores ; still they may be 

 supposed to represent the phloem. The axile bundle extends to close beneath the 

 meristem of the flat apex of the stem, and developes further in proportion as new leaves 

 appear. It arises, as Hofmeister and Sachs perceived, from the sympodial union of the 



leaf-trace-bundles on the one side and 

 of the bundles which pass into the roots 

 on the other '. The entire axile bundle 

 is surrounded, except at the apex and 

 the points where the bundles from the 

 leaves and roots join it, by a cambium- 

 layer of narrow tabular cells, which by 

 their growth and division in the cen- 

 tripetal direction, towards the bundle, 

 and in the centrifugal, towards the cor- 

 tex, give rise to a number of new cells. 

 But the increase in the number of cells 

 is very unequal in the two directions ; 

 it is large in the centrifugal direction, 

 and consequently the cortex increases 

 considerably in thickness and its outer 

 layers turn brown and die off, and are 

 replaced each year by cells formed by 

 the cambium. The secondary cortex 

 thus formed consists entirely of paren- 

 chyma. The axile bundle on the other 

 hand adds but few layers to its thick- 

 ness through the activity of the cam- 

 bium, and it only rarely happens that 

 any of the cells of these layers become 

 tracheides. The structure of the leaves 

 varies, according as the plants live 

 beneath the water, in marshes or on 

 dry land. In the first case the leaves are long and conical, and are traversed 

 by four air-spaces divided into compartments by septa ; a weak vascular bundle 

 occupies their axis, and their epidermis has no stomata. In the second case 

 the general structure is the same, but they have stomata and hypodermal fibrous 

 strands. In the third case the epidermis is furnished with stomata, and the 

 basal portion of the dead leaves form a compact black coat of mail round the stem. 

 The fundamental tissue, which is not separated by a bundle-sheath from the single 

 bundle that traverses the leaf, forms according to Russow beneath the epidermis strands 

 of colourless sclerenchyma, as in Isoetes Hysirix, and a dark brown-walled sclerenchyma 

 which is the chief constituent of the sheathing portion of the leaf. The structure of the 

 vascular bundle which traverses the leaf is said by Russow to be collateral, as in the 

 Ophioglosseae and Equisetaceae ; the xylem is not surrounded by the phloem, but the 

 two simply lie side by side ; consequently Russow considers the layers of transparent 

 tissue lying next to the central xylem of the stem to be also phloem. 



Selag inctla iiuregiia/i/olu 

 stem, inagn. 150 t 



' The class of the Lycopodineae therefore shows two extremes ; the one in Psilotum, where the 

 leaves are few and small and there are no vascular bundles in the leaves but the elongated stem 

 forms a bundle of its own, the other in Isoetes, where the short stem produces no vascular bundles 

 but the large leaves produce one each. 



