AS AIDS IN THE DETERMINATION OF SPECIES. 13 



stalk is unaffected by the change, or at least not rendered symmetrical, so that in 

 every example investigated it does not form a complete circle, but a curve open 

 upwards, with the two ends approaching, but never meeting. 



A section of the base of the midrib of the young sessile leaf (Fig. 8) likewise 

 shows the curve open upwards, whereas a section of the young square stem of the 

 same plant (Fig. 9) exhibits a symmetrical quadrangle, corresponding with the 

 outline of the stem, and continuous right round. 



An attempt to account for the generally different distribution of the wood in the 

 stem and in the leaf-stalk is made by Herbert Spencer in his "Principles of Biology."* 

 After noticing that there is a direct relation between mechanical stress and the 

 formation of wood, and that the general arrangement in stems is that of a cylinder, 

 he says : — " While axes are on the average exposed to equal strains on all sides, 

 most leaves, spreading out their surfaces horizontally, have their petioles 

 subjected to strains that are not alike in all directions ; and in them the hard tissue 

 is differently arranged. Its transverse section is not ring-shaped, but crescent- 

 shaped ; the two horns being directed towards the vipper surface of the petiole. 

 That this arrangement is one which answers to the mechanical conditions, is not 

 easy to demonstrate ; we must satisfy ourselves by noting that here, where the 

 distribution of forces is different, the distribution of resisting tissue is different. 

 And then, showing conclusively the connection between these differences, we have 

 the fact that in petioles growing vertically and supporting peltate leaves — petioles 

 which are therefore subject to equal transverse strains on all sides — the vascular 

 bundles are arranged cylindrically, as in axes." 



That this is only a partial explanation is shown by the fact that in 

 many instances the cylindrical arrangement of the wood occurs where the 

 distribution of forces is different from that of the stem, as in the leaves of 

 fig and walnut, already mentioned. It is true, however, that in all the 

 petioles of Eucalypts examined the crescentic form prevailed, and was not 

 appreciably interfered with by the twisting of the leaf-stalk. In some cases, such as 

 that of E.ficifolia, where the horns of the crescent may approach so very close as to 

 make the wood practically a cylinder, the outline was not cylindrical, and there was 

 less twisting of the leaf-stalk than in many other species. Indeed, in one petiole 

 of this species the wood formed a compressed hemisphere, the two horns abutting, 

 but as four central canals were present, this petiole was evidently somewhat 

 abnormal. 



The bundle will now be considered under three separate divisions, viz., hard 

 bast, soft bast, and wood. 



• Vol. II., p. 2G1. 



