3l8 PR IMA R}' ARRANGEMENT OF TISSUES. 



tracheie, consisting of several or many rows as seen in cross- section, without inter- 

 calated cells, occur in the xylem of thick bundles, as in the Marattiaceae, Osmun- 

 daceas, and Ophioglossese ^, and in the bundles of the leaf of Yucca, the stem of 

 Fritillaria '-, &c., to be described below. 



The external surface of a vascular bundle is marked off from the surrounding 

 non-equivalent tissue in various ways, and not uncommonly in such a manner that 

 the surrounding parenchymatous elements pass over directly and quite gradually into 

 those which lie within the bundle itself, between the tracheae and the sieve-tubes. 

 A smooth^ sharp, external boundary is then not present at all, although the bundle 

 itself, in so far as it consists of its essential elements, always stands out sharply. 

 More frequent no doubt is the complete or pardal limitation of the bundle by means 

 of a distinct sheath, strand-sheath, or bundle-sheath ^, in the sense of the word fixed in 

 general at p. 6. This occurs, firstly, in the form of the endodermis (p. 121), or other 

 specialised parenchymatous layers (Chap. IX) ; secondly, in the form of strands or 

 layers of sclerotic fibrous cells, or especially sclerenchymatous fibres, which border the 

 bundle on one side or encircle it all round. All these sheaths are as a rule limited 

 on the inside towards the bundle with the same sharpness as on the outside towards 

 the non-equivalent surrounding mass ; they can therefore just as well be assigned to 

 the bundle itself as to its surroundings, its boundary being fixed either at the external 

 or internal surface of the sheath. The latter is the usual and expedient practice in the 

 case of parenchymatous, and particularly of endodermal sheaths, especially on the 

 ground of developmental phenomena in almost all roots ; yet it must not be left out 

 of consideration that in the case of the endodermis of most bundles in Fern-stems 

 the history of development rather leads to the opposite result. Cf Sect. 106. 



As regards the sclerenchymatous strands and sheaths which accompany the 

 bundle longitudinally, it has long been customary to assign them to the vascular 

 bundles, or, as was done by Nageli, to regard them at least as essential concomitants 

 of the latter, and to call the united strand formed by them and the vascular bundle 

 the fibro-vascular strand. Both from former sections, and further from the general 

 comparative consideration of the distribution of sclerenchyma (Chap. X), it follows 

 that these fibrous sheaths and strands do not strictly speaking belong to the vascular 

 bundles, but to a special system of tissue, which may or may not have a common 

 course with the latter over certain tracts. 



In spite of all these theoretical considerations, the anatomical treatment of the 

 vascular bundles cannot by any means leave sheaths of whatever kind unregarded 

 where they occur. 



The elements of the vascular bundle are, as far as investigations reach, almost 

 everywhere and always in uninterrupted connexion, both among themselves and with 

 those of the surrounding sheaths. The only not uncommon exceptions are that the 

 xylem, especially in collateral bundles, shows intercellular spaces containing air at its 

 inner edge, and that spaces containing secretions lie in the outer regions of the 

 bundle. Cf. Chap. XIII. 



* Compare Russow, Vergl. Unters. p. 117. 

 " Von Mohl, Palm. Struct. Tab. G. Fig. 1 1. 



^ C. H. Schultz, Die Cyclose, /. <'. (p. 192) p. 246. — Sachs, Textbook {2nd Eng. ccL), p. 124. — 

 Compare also Russow, Vergl. Untersuch. 



