ORIGIN OF COLLATERAL STRANDS 363 



centric strand of the stem was about to be transformed into a circle 

 of separate bundles. When the radial extensions of the pith broke through 

 the leptome, the vascular cylinder became finally resolved into a ring of 

 isolated collateral strands, with external leptome. 



The disposal of the collateral vascular bundles in a single ring, 

 which is characteristic of Equisetum, of the Gymnosperms and of most 

 Dicotyledons, is evidently a more primitive condition, than the system 

 of concentric circles that occurs in some Dicotyledons, or the " scattered " 

 arrangement which prevails among Monocotyledons. The more detailed 

 discussion of these matters must be reserved for a subsequent section 

 (IV., A). For the present it will suffice to remark, that these aberrant 

 types of arrangement were doubtless in the first instance evolved with 

 reference to special physiological requirements ; it would consequently 

 be very unwise to regard all the bundles, in such abnormal types of 

 vascular structure, as necessarily derived from a single ancestral central 

 cylinder. The vascular system is, in fact like other anatomico- 

 physiological systems by no means invariably homogeneous in a 

 phylogenetic sense. When once the idioplasm has developed " deter- 

 minants " corresponding to collateral vascular bundles, conducting 

 strands of this type may be formed quite independently of the original 

 vascular cylinder, at any point where a physiological demand for these 

 structures happens to arise ; such accessory bundles, which may take 

 the form of medullary or cortical strands, or of anastomoses in the 

 mesophyll or at the nodes of stems, must be regarded as entirely new 

 departures from a phylogenetic point of view. 



As has been explained above, the orientation of the collateral 

 bundles in stems is such, that the leptome portions are next the 

 periphery, while the hadrome-strands face the centre. This arrange- 

 ment is determined by two factors, which both act in the same 

 sense. In the case of [horizontally extended] leaves, physiological 

 considerations render it necessary that the hadrome should be dorsal, 

 and the leptome ventral, in position. If a leaf-trace bundle of this 

 type enters the stem without torsion, the customary arrangement 

 follows as a matter of course. There is, however, a phylogenetic 

 factor, which tends to produce the same results. If, namely, 

 the relative positions of hadrome and leptome remain unaltered 

 after the disruption of the primitive hadrocentric central cylinder, the 

 resulting collateral bundles will naturally have their leptome portions on 

 the outer side. This purely morphological explanation does not, of course, 

 imply that the characteristic orientation of the axial bundles is altogether 

 devoid of physiological significance. Among Dicotyledons and Gymno- 

 sperms, at any rate, this orientation seems almost obligatory in view of 

 the subsequent occurrence of secondary thickening; moreover, since stems 



