354 CONDUCTING SYSTEM 



support ; here the pith is sclerotic and devoid of vascular elements, 

 while both the vessels and the sieve-tubes of the characteristic vascular 

 " rays " are of relatively small calibre. In the absorbing roots, on the 

 contrary, which grow vertically downwards and finally enter the soil, 

 the width of the central cylinder is equal to one-half of the total 

 diameter, while the ordinary hadrome- and leptorne-strands are supple- 

 mented by numerous wide vessels and sieve-tubes scattered throughout 

 the pith. 



Radial bundles also occur in the stems of Lycopodiaceae and in 

 the stolons of Nephrolcpis. In the upright species of Lycopodium, the 

 hadrome takes the form of radiating plates, which meet at the centre of 

 the stem. In those forms, on the other hand, which have prostrate 

 dorsiventral shoots, the hadrome is broken up into a number of separate 

 horizontally extended ribbon-shaped tracts. The more or less incurved 

 margin of each tract consists of annular and spiral tracheides, whereas 

 the central portion is entirely made up of scalariform elements. Each 

 tracheidal tract is enveloped in a layer of hadrome-parenchyma. The 

 intervals between adjacent hadrome-tracts are occupied by the leptome. 



A collateral vascular bundle consists of a hadrome- and a leptome- 

 strand, which run side by side in actual contact with one another. If 

 the hadrome is enclosed between two leptorne-strands, the bundle is 

 termed bicollateral. Collateral bundles predominate in the leaves and 

 stems of Angiosperms and Gymnosperms, and occur also in the stem of 

 Equisctum ; in most Ferns the smaller foliar bundles conform to this 

 type. In stems the orientation of the collateral bundles is usually 

 such that the leptome is external. A corresponding arrangement pre- 

 vails in dorsiventral leaves, the leptome facing the abaxial and the 

 hadrome the adaxial side. 



Collateral bundles exhibit less diversity of cross-sectional outline 

 than concentric strands. Most often they are approximately circular, 

 elliptical or oval ; but they may also be laterally compressed or ribbon- 

 shaped, as in the leaves of Hypochaeris, Pandanus, Hyacinthus, the 

 Scitamineae, etc. As a rule, the hadrome and leptome are combined in 

 such a way that the composite bundle possesses only a single plane of 

 symmetry. 



As regards minor details, the construction of collateral bundles is 

 exceedingly variable. A very well-defined type, which occurs in a great 

 many Monocotyledons, is characterised by the V- or Y-shaped cross- 

 section of the hadrome-strand, the base of the V being occupied by a 

 few spiral or annular vessels, or sometimes by an intercellular passage, 

 while the distal end of each arm is marked by a single very wide vessel 

 with spiral or closely reticulate thickenings (Fig. 143). The space 

 between the arms is either taken up by the leptome-strand or else filled 



