750 PART IV.-T-THE PHYSIOLOGY OF PLANTS. 



An exact coincidence between the direction of the long axis of the member 

 and that of the incident rays is, however, not always attained in nature, on 

 account of the antagonistic action of other directive influences. This point is 

 more fully discussed on p. 757. 



It must be mentioned that, inasmuch as there are no radial 

 members which are both heliotropically irritable and capable of 

 performing movements of variation, all that is here said refers to 

 growing radial members. 



In illustration, the case of a radial member which has been 

 grown in the dark may be taken, and it may be assumed to 

 be vertical. Light is allowed to fall upon it from one side ; the 

 effect is a gradual curvature of the member, as it continues to 

 grow, so that its long axis comes to coincide more or less nearly 

 with the direction of the incident rays. 



But the curvature may be in one of two directions ; it may be 

 either such that the apex of the member comes to point towards 

 the source of light, or such that it points in the opposite direction. 

 When the former is the case the member is said to be positively 

 \heliotropic ; when the latter, it is said to be negatively heliotropic. 



The nature of the curvature, whether positive or negative, 

 depends upon the specific irritability of the member. Thus, gene- 

 rally speaking, primary shoots, including such forms as the stems 

 of Chara and Nitella, the peduncles of flowers, the stipes of the 

 larger Fungi, and the gonidiophores of Moulds, as also radial 

 leaves such as those of the Onion, are positively heliotropic. 

 j Negative heliotropism has been observed in many roots, especially 

 aerial roots, and in the root-hairs of Marchantia. W^ith regard to 

 shoots, the hypocotyl of Viscum, the Mistletoe, is negatively helio- 

 tropic. ^ 



Although the relation between the external symmetry of the 

 member and its heliotropic irritability is generally that indicated 

 above, yet there are exceptions : all dorsiventral members are dia- 

 heliotropic ; but not all radial members are positively or negatively 

 heliotropic, for some of them are diaheliotropic. It seems that 

 continual exposure to intense light falling on one side induces at 

 least physiological dorsiventrality in some radial members (e.g. 

 shoots of Ivy and Tropoeolum). 



It is frequently stated that exposure to intense light causes a reversal of 

 heliotropic properties in radial members ; for instance, that a shoot which is 

 positively heliotropic in moderately intense light becomes negatively heliotropic 

 in very intense light. The true explanation of such cases is probably this, that 



