888 THE SHAPES OF HORNS [ch. 



a tendency long ago discussed by de Candolle and investigated by 

 Palm, H. von Mohl and Dutrochet*. This tendency to revolution — 

 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) acquires its spiral twist, and is due, in like 

 manner, to inequahties in the rate of growth of the growing stem : 

 with this difference between the two, 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 phe- 

 nomenon due to osmotic pressure. The particular stimulus to which 

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

 been ascribed can hardly be discussed here; but it was hotly 

 debated fifty years ago and for many years thereafter, the point 

 at issue being no other than whether direct physical causation, or 

 the Darwinian concept of fitness or adaptation, should be invoked 

 as an "explanation" of biological phenomena. The old Natur- 

 philosophie had been inclined to look for spirals everywhere, and to 

 attribute them to very simple causes: "Man wird nicht gross irren" 

 (said Okenf) "wenn man sagt, alle Pflanzen entstehen als Spirale, 

 und zwar weil sie feststehen und ein End gegen die Sonne kehren, 

 die taglich einen Spiralgang um sie ihacht, u.s.w." When de 

 Candolle saw a shoot curve under the influence of light (by helio- 

 tropism, as we are told to call it), he was content to regard the 

 curvature as the result of difi'erent rates of growth on one side 

 or other of the shoot, and these in turn as the direct result of 

 differences of illumination. But by the Darwins, father and son, 

 and by Sachs and by the Wiirzburg school, the curvature was 

 ascribed to "irritability," a "stimulus" on one side of the shoot 

 being followed by a "motor-reaction" on the other. The curvature 

 was thus taken to be a "response" to external stimuli (such as light 

 and gravity); and stimulus and response were supposed to have 



* Palm, Ueber das Winden der Pflanzen, 1827; H. von Mohl, Bau und Windert 

 der Ranken, etc., 1827; R. H. J. Dutrochet, Sur la volubilite des tiges de certains 

 vegetaux, et sur la cause de ce phenomene, Ann. Sc. Nat. {Bot.), ii, pp. 156-167, 

 1844, and other papers, 



t I sis, I, p. 222, 1817. 



