IRRITABILITY. 429 



negatively heliotropic, but Brefeld has been unable to con- 

 firm this observation. With regard to shoots, the hypocotyl 

 of Viscum (Mistletoe) is negatively heliotropic (Dutrochet, 

 Wiesner). It may be also mentioned here, as a matter of 

 historical interest, that the discovery of negative heliotropism 

 was made by Knight in his observations on the tendrils of 

 Vitis and Ampelopsis. The consideration of their heliotropic 

 properties will be deferred until we come to treat of dorsi- 

 ventral organs (p. 443). 



It is stated that some organs exhibit at one time positive, 

 and at another negative, heliotropism. Thus, according to 

 Sachs, the internodes of Tropceolum majus when young are 

 positively heliotropic, whereas when they are older they are 

 negatively heliotropic, and this is true also of the hypocotyl 

 of the Ivy. Wiesner has observed an apparent change in 

 heliotropic properties in the stems of Galium versum and 

 Mollugo, and in radial shoots of Cornus mas and sanguinea. 

 The same has been observed in the peduncles of Erodium 

 cicutarium and in those of Taraxacum officinale by Vochting. 

 In all these cases the organs exhibit positive heliotropism 

 when exposed to feeble light, and negative heliotropism when 

 exposed to strong light. But it is a question if we have 

 really to do here with a reversal of heliotropic properties. We 

 know already (supra, p. 425) that exposure to intense uni- 

 lateral illumination causes a change in the nature of the 

 primary shoots of Tropaeolum and of the Ivy; from being 

 radial they become dorsiventral. It is highly probable that 

 in all these cases the apparent reversal of heliotropic pro- 

 perties is simply the expression of such a change of nature. 

 We will further consider this subject when we are dealing 

 with the influence of light upon dorsiventral organs. 



But there are cases of a reversal of heliotropic properties 

 which is apparently not due to a variation in the intensity of 

 the light, though no precise statement is made on this point 

 in the account of the observations, but is dependent upon the 

 biological conditions of the organs. Hofmeister states that 

 the floral peduncle of Linaria Cymbalaria is positively 

 heliotropic, but that when the fruit has replaced the flower 



