STRUCTURE OF COLLATERAL BUNDLES. 



337 



their course through the leaf, the thickness of the individual portions diminishes, 

 especially the external portion of the xylem. In many species, at any rate, the bundles 

 in the cross-section of the leaf-stalk are generally arranged in the figure of an 

 inverted O, with the limbs directed towards the upper side. In the constricted part 

 of the figure either they are separate, the constriction being open ; or they approach 

 one another in pairs, the inner vascular portions of each pair being turned towards 

 one another, and the scalariform vessels of each in uninterrupted connection over a 

 broad surface. In Zamia longifolia one such coupled bundle is present near the 

 middle of the cross-section ; in Dion there are about six of them forming a row 

 vertical to the surface of the leaf\ Other species of Zamia show, according to 

 Mettenius, a less regular grouping of the bundles and of their connections. 



In Cycas revoluta t-he bundle is encircled by a thick-walled sclerotic sheath, 



FIG. 159. — Cycas revoluta (223). Longitudinal section througli a similar bundle ot the same leaf-stalk, in the 

 direction r— r, Fig. 15S. The letters have the same meaning as in that figure. « reticulated tracheide with narrow 

 slits ; the fibres of the narrowest (first) spiral tracheide are distorted. 



sharply limited both inside and out, which consists of short, wide, angular elements 

 [c c), with pitted or narrowly reticulated walls, and cavities often filled up by large 

 crystals of Calcium oxalate. In the other species investigated, there is no sheath 

 which can be sharply distinguished from the surrounding parenchyma ; at most 

 some of the sclerenchymatous fibres, which are scattered through the whole tissue, 

 stand around the periphery of the bundle. 



According to Russow ^, the feeble leaf-bundles of Isoetes closely approach those 

 of the Cycadese as regards structure. They are collateral and their orientation is 

 normal. Their xylem consists of narrow prismatic parenchymatous cells, with ,some 

 narrow, spiral, and reticulated tracheides standing between them. The primitive 

 elements appear, according to Russow, on the boundary of the phloem, while the 

 others, which are nearer the inner edge of the bundle, attain their development later. 

 On the inside of the phloem thin-walled prismatic elements can be distinguished, but 



' Mettenius, I.e. p. 573, Taf. i, fig. 10. 



/.cpp. 140, 155. 



