328 PRIMARY ARRANGEMENT OF TISSUES. 



2-4 in each cross-section — are formed, between delicate narrow cells; they are 

 elongated, and ranged in longitudinal rows, one above the other, like vessels. One 

 of these rows becomes expanded, according to van Tieghem's description apparently 

 passively, to form a passage surrounded by narrow cells, the thickenings on its wall 

 disappearing in places. The very oblique, fibrously thickened (unperforated ?) end- 

 surfaces, with which the articulations are in contact one with another, are persistent ; 

 in cross-sections, therefore, the passage often appears divided by a septum into two 

 unequal portions. The other tracheides, lying partly inside, partly outside the dilated 

 one, remain narrow and delicate, with annular or spiral fibrous thickening. 



In the scape and leaf-stalk the passages increase in width with their distance 

 from the periphery, the outermost bundles have only a narrow row of tracheides in 

 place of them. They accompany the bundles into the lamina of the leaf, extending into 

 the thick strand into which the bundles are united at its apex : here they lie close 

 side by side in great numbers, forming the often-described water canals of the 

 leaves of Aroideae. In the leaves also they are accompanied by rows of tracheides 

 wliich are not dilated, with which transverse anastomosing branchlets are everywhere 

 connected, the petiole not excepted. 



In the thicker bundles of the leaf of Sparganium ramosum a wide passage is 

 produced, according to Franks in the same manner as among the Aroideae. The 

 contents of the passages consist of air and watery liquid, while among the Aroideae 

 latex containing tannin is also present in places. Comp. p. 188. 



The sheaths of collateral bundles consist either of simple parenchyma, or, rarely, 

 of the form described as endodermis ; or, lastly, and indeed in the majority of col- 

 lateral bundle-trunks, of strands of sclerenchymatous or collenchymatous fibres, which 

 accompany the bundles, w^hether it be as a tube encircling the whole bundle, or as 

 a strand which partly surrounds the circumference of the bundle. In the latter case 

 it rarely borders exclusively or principally on the xylem, but usually on the phloem, 

 or only on its outer edge. These sheaths and accompanying strands are to be 

 separated from the bundle itself, 'and considered in the following chapters. Here 

 we have only to mention that those which consist of sclerenchymatous fibres may 

 not uncommonly border immediately on the elements of the bundle, and even insert 

 their own elements between the latter, so that the limitation, especially as seen in 

 cross-sections, ceases to be clear, and the arrangement of the specific parts of the 

 vascular bundle is often influenced in a peculiar manner. 



A gradual transition from the sclerenchymatous elements of the sheath to the 

 cells of the xylem very often takes place in cases where the latter are provided with 

 strongly thickened and lignificd membranes, e.g. in stout bundles of Monocotyledons. 

 (Fig. 150.) 



The sclerenchymatous sheath is generally very sharply limited on the side of 

 the phloem ; the latter lies as a uniformly thin-walled mass of tissue between the 

 sheath and the xylem. It happens in rare cases that here also the sclerenchyma of 

 the sheath penetrates deeply into the phloem, and is continued as far as the thick-walled 



d. Pfl. Wiener Acarl. Sitzgsber. Bd. XXVIII. p. iii.— De la Rue, Botan. Zeitg. 1866, p. 816.— Van 

 Tieghem, Struct, des Aroidees, I.e. 

 * Beitr. p. 137. 



