688 



PART IV. THE PHYSIOLOGY OF PLANTS. 



sensitive to the directive action of light and of gravity and, in 

 the course of their growth they take up a definite position 

 termed, on account of the predominating influence of light in 

 determining it, the fixed light-position. The response of the dor- 

 siventral leaf to the directive action of gravity, is generally one 

 of diageotropism, that is it places its blade horizontal, with the 

 ventral surface uppermost; and similarly, its response to light is 

 to expose the upper surface of its blade at right angles to the 

 direction of the incident rays (diaheliotropism) . The response of 

 the isobilateral and of the radial leaf to the action of gravity is one 



FIG. 470 (after Darwin). Shoots of Desmodium gyrans, with trifoliolate leaves: A leaves 

 in diurnal position ; B in nocturnal position. 



of negative geotropism, so that they grow erect ; and to light, one 

 of positive heliotropism, as they tend to direct their apices towards 

 the source of light. 



Changes in the external conditions act as stimuli, which, in many 

 cases, induce a movement of the foliage-leaves involving change 

 of position : most frequently these movements are performed 

 by growing leaves, but also sometimes by adult leaves with a 

 permanent motile mechanism. They have been observed in the 

 growing leaves (and cotyledons) of many plants (e.g. Cheno- 



