LEPTOCENTRIC BUNDLES 365 



concludes, that the dorsal strand, in which the sieve-tubes are all 

 comparatively narrow, supplies the developing tissues with plastic 

 material, but becomes inactive when the leaf is fully grown; the ventral 

 strand, on the contrary, serves just like the single leptome-strand of a 

 normal collateral bundle to collect and carry away the nitrogenous 

 compounds manufactured in the leaf. 



The leptocentric bundle that is, the concentric bundle with central 

 leptome is connected with the collateral type by a series of inter- 

 mediate forms. The frequent occurrence of leptocentric strands in 

 Monocotyledonous rhizomes, and in the medullary tissue of certain 

 Dicotyledons, gives a clue to the direction in which one has to look for 

 the physiological significance of this type of vascular structure. Such 

 bundles, in short, occur principally in storage-organs and -tissues, and 

 more particularly in those which are concerned with the storage of 

 carbohydrates and of water, or of the former alone. The extension of 

 the hadrome in the form of a sheath completely surrounding the leptome, 

 is apparently designed, as Strasburger has pointed out, to bring the 

 greatest possible number of vessels and tracheides into direct contact 

 with the surrounding storage-tissue. As will be explained later on 

 (Ch. XIV., ii. C. 2), the water-conducting channels of many woody plants 

 are utilised for transporting carbohydrates as well as water, when the 

 buds are unfolding in spring ; it is therefore quite conceivable, that the 

 tracheal elements of leptocentric bundles likewise perform this twofold 

 conducting function, in which case their peripheral disposition within 

 the individual bundles- must undoubtedly facilitate the depletion of 

 the adjacent storage tissues. In any case, this argument is applicable 

 to the cases in which leptocentric strands are associated with water- 

 storing tissues. Certainly Strasburger's theory of the leptocentric 

 (amphivasal) type of vascular structure, if not finally established, seems 

 at any rate very plausible. 



C. THE PERIPHERAL BUNDLE-ENDS. 



The numerous ultimate ramifications of the vascular system usually 

 terminate in the photosynthetic mesophyll ; this arrangement is quite 

 in accordance with expectation, since the latter tissue on the one 

 hand absorbs the largest proportion of the transpiration current, and on 

 the other hand supplies the bulk of the plastic material which has to 

 be transported in the opposite direction. Both these circumstances 

 are reflected in the construction of the bundle-ends, which in some 

 respects show a progressive simplification of structure, but at the same 

 time develop certain features that are absent from the rest of the 

 vascular system. 



The hadrome -portion of such a bundle-end is usually composed of one 



