622 PLANT RESPONSE 



example, runners of Lysimachia Nummularia, the result was 

 a concavity of that side ; and when strong light was applied 

 on the upper surface, the result was still the concavity of the 

 normally shaded, and now distal, side. He obtained similar 

 results also with midribs of leaves. Again, in the case of 

 Marchantia thallus, it was found by Frank and Sachs that it 

 was the normally shaded side which became concave, whether 

 light was applied above or below. 



De Vries explained the curvature away from light, when 

 the dorsal surface was illuminated, by the assumption that 

 the organ was negatively heliotropic ; but the concave 

 curvature induced when the lower surface of the same organ 

 was illuminated necessitates the description of that surface, 

 at least, as positively heliotropic. Thus we are driven to 

 assume that the two surfaces of the organ are endowed at the 

 same time with two opposite heliotropic properties, the obvious 

 impossibility of which has led to the idea that in this class of 

 phenomena we have not to deal with the directive action of 

 light at all. 



In the case of certain pulvinated organs I shall be able to 

 show an exact parallel to these phenomena — that is to say, an 

 induction under light of concavity in the lower surface, whether 

 light be applied from above or below. This being so, it is 

 clear that the only theory of the phenomena which could be 

 satisfactory would be one which would apply equally to both. 



Closely connected with the same inquiry is the pheno- 

 menon of para-heliotropism, or the so-called diurnal sleep. 

 Under the intense illumination of midday, the leaves or 

 leaflets of certain plants take up positions which outwardly 

 resemble those which they adopt at night. In some cases 

 again, in which the normal daylight position is outspread, and 

 the nocturnal one of downward folding, the effect of light at 

 midday on the leaflets is to induce a folding upwards. 

 These phenomena have not yet been satisfactorily explained. 

 Darwin suggested that such habits had been acquired for the 

 special purpose of avoiding too intense an illumination. I 

 shall be able, however, to offer a simple and inclusive ex- 



