442 LECTURE XVII. 



of light, whereas in intense light their long axes are at right 

 angles to the direction of the incident rays. 



Finally, dorsiventral leaves are usually more or less nearly 

 horizontal when growing exposed to light, whereas, when 

 etiolated, they frequently tend to place themselves rather in a 

 vertical than in a horizontal plane. This is especially marked 

 in the case of radical leaves (Frank). 



Some more definite information is afforded by the observa- 

 tions which have been made on the behaviour of dorsiventral 

 organs when their dorsal (morphologically upper) and ventral 

 (morphologically lower) surfaces are respectively exposed to 

 light. With regard to dorsiventral shoots, de Vries found, 

 in the case of runners of Polygonum aviculare, Lysimachia 

 Nummularia, and others, that if such a shoot be placed verti- 

 cally and light be allowed to fall upon its dorsal surface, the 

 shoot curves away from the source of light. When on the 

 contrary, the ventral surface is exposed to light, the shoot 

 curves towards the source of light. He obtained similar results 

 when he exposed the mid-ribs of leaves, freed from the meso- 

 phyll, under similar conditions. When the dorsal surface was 

 exposed to light, the curvature, if any, was always such that 

 the dorsal surface became convex to the source of light; when 

 the ventral surface was exposed, there was in all cases a well- 

 marked curvature such that the ventral surface became concave 

 to the source of light. Similarly, both Frank and Sachs have 

 observed that when the dorsal surface of a Marchantia-thallus 

 is exposed to light, the thallus becomes convex towards the 

 source of light, whereas when the ventral surface is exposed, 

 it becomes concave. 



De Vries attributes the curvature away from the source of 

 light to the possession of negative heliotropic properties by 

 these organs, an opinion which is shared by Frank as far as 

 Lysimachia Nummularia is concerned. But the concave 

 curvature when the ventral surface of the organs was exposed 

 to the light remains unaccounted for. In order that this expla- 

 nation of the phenomena may be consistent throughout it is 

 necessary to assume that the two surfaces of the organ, in 

 these cases, are endowed with different heliotropic proper- 



