CONCENTRIC VASCULAR STRANDS 349 



doubt in itself furnished a sufficient reason for its juxtaposition to the 

 comparatively resistent hadrome ; in any case, this association most 

 probably first took place at a stage of evolution antecedent to the 

 differentiation of a special mechanical system (which the author regards 

 as a relatively recent development). According to this view, the now 

 prevalent fibrous sheaths were added at a later period for the sake of 

 additional protection. In the case of photosynthetic organs, a further 

 advantage accrues from the association of hadrome and leptome, inas- 

 much as in these circumstances a single set of intermediary tissues 

 suffices to connect the photosynthetic tissue with all the three types of 

 conducting tissue. If hadrome, leptome and conducting parenchyma all 

 formed separate strands, the efferent arrangements would have to be 

 greatly amplified and elaborated. 



2. Various types of composite conducting strand. 



It is customary to recognise three principal types of vascular 

 bundle, which are defined in accordance with the relative position of 

 hadrome and leptome, namely, the concentric, the radial and the 

 collateral types. It must, however, be clearly stated at the outset, that 

 these different forms of vascular bundle are not all homologous or 

 phylogenetically equivalent structures. 



A vascular strand is described as concentric, when one of the two 

 principal components occupies its centre and is surrounded by the 

 other as by a sheath. If the hadrome is central and the leptome 

 peripheral, the bundle may be termed hadrocentric (amphicribral). 

 Bundles of this type prevail in the stems and leaves (the smaller bundles 

 being excepted in the latter case) of Ferns, and also occur in certain 

 Dicotyledons. The opposite or leptocentric (amphivasal) condition is 

 exemplified by the leaf-trace bundles in many Monocotyledonous 

 rhizomes (such as those of Cypcrus aureus, Papyrus, Acorus Calamus, 

 Iris germanica, etc.), and by the medullary strands of a number of 

 Dicotyledons (Piperaceae, Phytolacca dioica, spp. of Rheum, Rumex, 

 Drosera, Geranium, Arcdia, Begonia, Statice, Campanula, Seorzonera, etc). 



In their simplest form, the hadrocentric conducting strands of 

 Ferns are circular or elliptical in cross-section (Fig. 139); more often, 

 however, they assume a flattened or ribbon-like shape, while they may 

 in addition be folded so as to resemble a V, U or X when seen in 

 transverse section. The outline of the hadrome, which, as already 

 stated, occupies the centre of the strand, either follows the outline 

 of the entire bundle more or less closely, or else displays a symmetry 

 which is cpuite independent of the latter. In the smaller bundles the 

 hadrome consists almost exclusively of spindle-shaped scalariform 

 tracheides, only the narrow first-formed elements being annular or 



