444 LECTURE XVII. 



any circumstances, the dorsal surface becomes concave when 

 light falls directly upon it. Sachs mentions that the leaves of 

 a plant of Tropceolutn majus grown in a window were positively 

 heliotropic. De Vries found that neither dorsiventral leaves 

 nor shoots ever exhibited positively heliotropic curvature when 

 the light fell on the dorsal surface, but he is nevertheless 

 inclined to attribute positive heliotropism to the former. 

 Wiesner states that he has frequently detected positive helio- 

 tropism in leaves. But these statements as to the positive 

 heliotropism of leaves are open to criticism. In the first place 

 the apparent positive heliotropism may reside, not in the 

 dorsiventral lamina, but in the petiole' which may be radially- 

 organised. And secondly, it may be that when it is exhibited, 

 as in the cases adduced by Wiesner, in feeble light, it is simply 

 the expression of a partial etiolation, a condition which, as we* 

 know, prevents the expansion of the blade so that its upper 

 surface remains concave. A form of positive heliotropism, but 

 not the one which we are considering here, has indeed been 

 observed in dorsiventral organs. Sachs has pointed out, for 

 instance, that when a leaf of Fritillaria imperialis was so 

 placed that the incident rays fell upon one lateral margin, this 

 margin became concave so that the blade assumed a sickle- 

 shape : de Vries has found the same to occur in a number of 

 cases (Rhus typhina^ Ailanthus glandulosa, Spircea sorbifolia, 

 etc.), and Wiesner has made similar observations on the leaves 

 of Campanula persicifolia and on the cotyledons of the Silver 

 Fir (A bies pectinata] . 



We come then to the conclusion that when either the dorsal 

 or the ventral surface of a dorsiventral organ is exposed to light 

 it exhibits neither negative nor positive heliotropism, but only 

 photo-epinasty (or photo-hyponasty). We must be careful not 

 to regard photo-epinasty as belonging to the category of helio- 

 tropic phenomena, that is, as being a manifestation of the direc- 

 tive influence of light, for the same effect is produced whether 

 the dorsal or the ventral surface of the organ is the one upon 

 which the incident rays directly fall. And if any further proof 

 of this is wanted, it is afforded by Detmer's observation that 

 the lamina of a leaf (he experimented with the cotyledons of 



