DORSIYENTRAL INFLORESCENCES 137 



towards the other side, as happens in Melampyrum pratense and M. 

 sylvaticum, Scutellaria peregrina ; in this way the flowers at flowering 

 time may no longer be enveloped by the bracts. 



I have no doubt that the unilaterality of the inflorescences just 

 mentioned is in many cases caused by external factors, and one is dis- 

 posed to agree with Vaucher 1 that light has a predominating influence. 

 He says for example regarding Melampyrum : ' This direction of the 

 flowers to the illuminated side is so marked that Melampyrum sylva- 

 ticum. which only grows in the middle of a wood, enables one to 

 determine which side of the wood receives most light. I have 

 often noticed the flowers of one plant directed differently according to 

 their elevation on the plant. . . .' Noll's investigations 2 however show 

 that the influence of light is not conspicuous everywhere. At the edge 

 of a wood the inflorescences of Digitalis all turn the side bearing 

 flowers towards the light, but even if the plant be illuminated on 

 all sides the inflorescences are still sharply unilateral. The axes of 

 inflorescence show from a certain age onwards a curvature of nutation and 

 therefore become overhanging, the flower-stalks are positively geotropic, 

 the flowers must therefore be turned to one side and are, on lateral 

 inflorescences, necessarily turned away from the chief axis. In unilateral 

 illumination the axes of inflorescence curve in a positively heliotropic manner. 

 The same is probably the case in Convallaria Polygonatum and others, 

 and in the conspicuous unilateral inflorescences of species of Scutellaria :! . 

 In this latter species the flowers on plants which are strongly illuminated 

 from one side are all turned outwards, and a torsion of the internodes of 

 the inflorescence takes place alternately in an opposite direction whereby 

 the bracts of the flowers which originally stood in cross-pairs come to lie 

 almost in two rows. The axes of inflorescence also hane over through 



t> o 



' spontaneous nutation,' and the direction of the overhanging determines 

 that of the unilaterality as in the case of Digitalis, but the flower-stalks 

 are positively geotropic. As the plagiotropous lateral shoots springing 

 from the axils of the leaves of the chief shoots bend away from the 

 chief axis, the flowers must all be turned to the outside. The total 

 result here then will depend upon the following factors: 



i. The assumption of an inclined position on the chief axis, which 

 I would designate simply as plagiotropous. 



' Vaucher, Histoire physiol. des plantes d'Europe, vol. iii, p. 543. See also Wiesner, Die helio- 

 trop. Erscheinungen im Pflanzenreich, p. 62. 



Noll, Uber die normale Stellung zygomorpher Bluten und ihre Orientierungsbewegungen zur 

 Erreichting derselben, in Arbeiten d. bot. Instituts in Wiirzburg, iii, p. 235. 



3 See regarding this Noll 1. c. and Kolderup Rosenvinge, L' Organisation polaire et dorsiventrale 

 des plantes, in Revue generale de Botanique, i. 



