GROWTH, IRRITABILITY AND MOVEMENT 159 



Although earth-roots in general show no phototropic response, 

 those of some plants, such as Mustard (Sinapis), show negative photo- 

 tropism, i.e. growth away from the source of light. As in geotropism, 

 we see that here again the phototropic response of these roots is the 

 converse of that characterising stems, a fact for which there is as 

 yet no agreed explanation. The aerial roots of Ivy and the tendrils 

 of Virginian Creeper [Ampelopsis) are also negatively phototropic, 

 a property of obvious significance in relation to their function (see 



also p. 217). 



Leaves are on the whole diaphototropic, placing themselves so that 

 their upper surface is presented at right angles to the incident light, 

 a position which secures a maximum incidence of light on the leaf, 

 and is obviously a very suitable one for photosynthesis. Under 

 natural conditions of growth, light usually falls on the leaves of a 

 plant from a variety of angles : it is the direction from which the 

 most light comes that determines the position which the individual 

 leaves take up during their development. Phototropic responses 

 of leaves are usually prominent in plants grown in dwelling rooms or 

 against a wall. The adjustments of leaf-position in response to the 

 direction of light are mostly due to growth-curvatures and torsions 

 of the petiole. In the case of plants growing in tropical or other 

 regions where they are exposed to very intense sunlight, leaves may 

 take up a quite different position, one in which the incidence of light, 

 especially of the strongest midday rays, is minimised. Some examples 

 of this are given later in the section on Xerophytes. A further 

 well-known instance is provided by the so-called Compass plants, 

 of which the European Lactuca scariola is an example. When this 

 plant is growing in a position subject to strong sunlight, the leaves, 

 though actually produced spirally on the stem, twist until they all 

 come to lie in a vertical plane running North and South, i.e. in the 

 position in which they will be least exposed to the hot midday sun- 

 light. In this way overheating of the leaves and excessive transpira- 

 tion may be avoided. 



As in geotropic responses, the type of phototropic response exhibited by 

 a particular organ may vary during its development. Thus the flower stalks 

 of Ivy-leaved Toadflax (Linaria) are positively phototropic during flowering, 

 but subsequently turn away from the light, with the result that the fruits 

 tend to be deposited in the crannies of some wall on which the plant usually 

 grows. Experiment has also revealed that the type of phototropic curvature 

 exhibited by an organ may vary with the strength of the light-stimulus. Thus 

 although positive phototropism is usually obtained with the coleoptile of 



