THE BILATERALITY OF APPENDAGES. 291 



persists ; although, again, the amount of eccentricity may be very 

 slight, and, when the bulk-ratio is small, quite inappreciable to 

 the eye. In the case of sporophylls, more especially stamens, 

 the approximation to a circular section in the developing prim- 

 ordium may be very close ; * but in the majority of foliar members 

 the structural bilaterality becomes increasingly exaggerated in 

 the form of the so-called dorsiventrality of the leaf, as these 

 members become specialised as assimilating laminae, exposing 

 the maximum surface to gaseous interchange. 



In fact, the primordia seen at the apex of a typical leafy shoot 

 are usually obviously bilateral from their first appearance, and 

 thus apparently flattened in a tangential direction; but as 

 previously indicated, their first appearance tells little of their 

 first inception. And, just as it has been shown that a uniform 

 cessation of growth at a certain stage in all the lateral members 

 may, as in the case of Ooleochaete (fig. 87), suggest the appearance 

 of a uniform growth-increment comparable to the effect of a 

 uniform growth-movement expressed as a uniform velocity ; or 

 again by the subsequent attainment of a constant bulk in the 

 lateral members may create a subjective impression of torsion- 

 spirals, intersecting as helices on a cylindrical axis where no torsion 

 exists; so this flattening of the members will necessarily produce 

 appearances to a certain extent suggestive of the action of a 

 strong compressing force. There is no need whatever to assume 

 that the first production of bilateral symmetry in a leaf-primordium 

 is caused by the stimulus of any mechanical pressure in the bud ; 

 it depends primarily on the actual mechanical construction of 

 the growing zone. The primordia which subsequently grow 

 according to their inherent dispositions are able at first to resist 

 all pressures of adjacent primordia ; and so long as these are 

 equally distributed along orthogonal paths of construction, the 



* An interesting example is afforded by the development of the sporophylls 

 of Clematis (G. integrifolia, Jackmanni, etc.), and a comparison with the 

 formation of the primary branches of the terminal umbel of such a form as 

 Heracleum giganteum. The former presents a system of leaf-members, the 

 latter a system of branches whose subtending bracts are suppressed ; but to 

 the eye the appearances presented by the two cases, and the shape of the 

 primordia and their contact-relations, are identical so far as can be judged. 



