506 LECTURE XIX. 



diaheliotropism, though it may at the same time curve 

 laterally away from the light in consequence of photo-epi- 

 nasty. Its plagiotropism, that is, the angle of deviation of 

 its long axis from the vertical, is clearly not affected. 



The manifestation of sensitiveness to light by these 

 organs, is, as we have seen, affected by the intensity of the 

 light. The light falling upon the organs must be of a certain 

 intensity in order that any perceptible plagiotropic effect may 

 be induced. For instance, we know that when negatively 

 geotropic dorsiventral organs grow in faint light, the effect of 

 gravity asserts itself by an upward curvature (p. 441). But 

 it does not follow that the most intense light produces the 

 greatest effect ; on the contrary, different organs seem to be 

 most sensible to light of different degrees of intensity. For 

 instance, the fixed light-position of most leaves, when fully 

 exposed to light, is such that their surfaces are horizontal, the 

 morphologically superior surface being directed upwards ; the 

 significance of this is, that it is in this position that the su- 

 perior surface of the leaf is perpendicular to the direction of 

 incidence of light of the appropriate intensity, which appears 

 to be, according to Wiesner (p. 447), the brightest diffuse 

 daylight. In some cases, as we have seen (p. 445), leaves 

 assume, either by curvature or by torsion round their long 

 axes, a vertical position under the same conditions of illu- 

 mination ; this position is doubtless also determined by the 

 direction of incidence of the light of that degree of intensity 

 to which the leaves are most sensitive. A striking instance 

 of this is afforded by the Compass-plants, alluded to in a 

 previous lecture (p. 446), in which, when fully exposed to 

 light, the surfaces of the leaves are vertical, this position 

 being determined by the oblique or horizontal rays of the 

 morning and evening sun. These plants with vertical leaves, 

 in fact, exhibit diaheliotropic irritability in a very high de- 

 gree, so much so, indeed, that their response to the directive 

 influence of light may entirely obliterate that to the action 

 of gravity. The leaves are, as a matter of fact, negatively 

 geotropic ; but, when the plants grow exposed to light, it 

 seems to be a matter of little importance what the direction 



