COMPOSITE CONDUCTING STRANDS 347 



namely, the fibrous cylinder and the collenchyma with protein 

 compounds. 



B. COMPOSITE CONDUCTING STRANDS. 172 



1. General considerations. 



In the great majority of cases, the different conducting elements are 

 associated to form composite strands. A typical conducting strand, in 

 the anatomico-physiological sense, therefore comprises tissues of three 

 different kinds. The protein-conducting elements, namely, the sieve- tubes 

 and companion-cells, and, it may be, also leptome-parenchyma (cambi- 

 form cells), form sometimes in conjunction with conducting parenchyma, 

 but more frequently alone the delicate leptome portion (Sieb-tcil, 

 Cribral-teil) of the strand. The water-conducting vessels and tracheides 

 constitute almost always in conjunction with conducting parenchyma 

 the resistant hadrome portion {Gefass-teil, Vasal-teil). Both hadrome 

 and leptome, therefore, but especially the former, comprise, in addition 

 to the characteristic components that serve for the conduction of water 

 and protein-compounds, elements of the nature of conducting parenchyma, 

 which may in short be termed hadrome- and leptome-parenchyma 

 respectively. The composite strands formed by the combination of one 

 or more hadrome- and leptome-strand with one another are known as 

 mestome- strands or vascular bundles. In most cases each vascular 

 bundle is enclosed within a bundle-sheath of one kind or another. 

 Sheaths of conducting parenchyma are characteristic of leaf-blades ; 

 starch- sheaths, which really represent sense-organs, are found in stems 

 and petioles ; endodermal sheaths (Schutzscheiden), finally, may occur in 

 stem, leaf or root. 



Before it was realised that the mechanical strands form an in- 

 dependent system, and that their frequent association with vascular 

 bundles is merely the result of physiological opportunism, a vascular 

 strand, the groups of wood fibres associated therewith and the sur- 

 rounding bundle-sheath, were collectively held to constitute a morpho- 

 logical unit, termed a fibro -vascular bundle. The phloem portion of 

 such a fibro-vascular bundle corresponds to the leptome together with 

 its fibrous sheath ; the xylem includes the hadrome with its associated 

 wood-fibres (libriform cells). If the leptome has no fibrous sheath, it 

 of course becomes synonymous with phloem ; where, as in most Mono- 

 cotyledons, no wood-fibres are developed, xylem is the exact equivalent 

 of hadrome. 173 



The adjoining table will help to explain the terminology employed 

 in the present work in referring to the various conducting elements, 



