436 LECTURE XVII. 



and enquire into the causes and nature of the curvature. It 

 must be premised that not all radial organs are heliotropic. 

 In addition to the peduncles already mentioned, the twining 

 internodes of climbing plants, and the stem of the Mistletoe, 

 are examples of shoots which are only slightly if at all helio- 

 tropic. Among roots, it appears that earth-roots are in most 

 cases not at all heliotropic, and this is also true, though excep- 

 tionally, of some aerial roots. The degree of heliotropic 

 irritability is also very different ; in some cases it is such 

 that, under appropriate conditions, the heliotropic curvature 

 is performed in spite of the opposing effect of other influences, 

 such as gravity, etc., whereas in other cases a curvature can 

 only be detected when the action of all other directive in- 

 fluences is as far as possible prevented. The differences in 

 heliotropic irritability are clearly illustrated by the fact, to 

 which allusion has been made more than once in this and in 

 the previous lecture, that the heliotropic effect is exhibited 

 by highly sensitive organs at an intensity of light which is 

 greater than that requisite to arrest growth, whereas in less 

 sensitive organs the heliotropic effect ceases to be manifest at 

 an intensity of light which is less than that requisite to arrest 

 growth. Another means of estimating heliotropic irritability 

 is afforded by the measurement of the angle made by the 

 axis of an organ with the direction of the incident rays when 

 the organ has ceased to curve heliotropically, the intensity of 

 the light and the other conditions being constant. The more 

 heliotropically irritable the organ, the more nearly will its 

 axis come to coincide with the direction of the incident rays. 



We may with advantage pause here for a moment to 

 consider the fact, implied by the statement just made, that 

 heliotropic curvature of an organ may take place without 

 growth in length. We are, it is true, considering the various 

 manifestations of irritability by growing organs, and yet we 

 here include one exhibited by organs which cannot, strictly 

 speaking, be said to be growing. Under ordinary conditions, 

 heliotropic curvature, as we shall soon see more fully, is 

 accompanied by growth in length ; it is a phenomenon of 

 growth in length, and is dependent upon the fulfilment of all 



