624 THE SHAPES OF HORNS [ch. 



horn. It is apparently on account of these minor corrugations 

 that, in such horns as the Highland ram's, where they are strongly 

 marked, the main St Venant effect is not nearly so well shewn as 

 in the smoother horns such as those of 0. Amnion and its immediate 

 congeners*. 



A further Note upon Torsion. 



The phenomenon of torsion, to ,which we have been thus 

 introduced, opens up many wide questions in connection with 

 form. Some of the associated phenomena are admirably illustrated 

 in the case of climbing plants; but we can only deal with these 

 still more briefly and parenthetically. 



The subject of climbing plants has been elaborately dealt 

 with not only in Darwin's books f, but also by a very large number 

 of earlier and later writers. In " twining" plants, which constitute 

 the greater number of "climbers," the essential phenomenon is a 

 tendency of the growing shoot to revolve about a vertical axis — 

 a tendency long ago discussed and investigated by such writers 

 as Palm, H. von Mohl and DutrochetJ. This tendency to revolu- 

 tion — "circumvolution," as Darwin calls it, "revolving nutation," 

 as Sachs puts it — is very closely comparable to the process by which 

 an antelope's horn (such as the koodoo's) grows into its spiral 

 or rather helicoid form ; and it is simply due, in like manner, to 

 inequalities in the rate of growth on different sides of the growing 

 stem. There is only this difference between the two cases, that 

 in the antelope's horn the zone of active growth is confined to 

 the base of the horn, while in the climbing stem the same 

 phenomenon is at work throughout the whole length of the growing 

 structure. This growth is in the main due to "turgescence," 

 that is to the extension, or elongation, of ready-formed cells 

 through the imbibition of water; it is a phenomenon due to 

 osmotic pressure. The particular stimuli to which these move- 

 ments (that is to say, these inequalities of growth) have been 



* The curves are well shewn in most of Sir V. Brooke's figures of the various 

 species of Argali, in the paper quoted on p. 614. 

 ► t Climbing Plants, 1865 (2nd edit. 1875); Power of Movement in Plants, 1880. 



J Palm, Ueber das Winden der Pflanzen, 1827; von Mohl, Ban iind Winden 

 der Banken, etc., 1827; Dutrochet, Mouvements revolutifs spontanes, C.B. 1843, 

 etc. 



