110 HELIOTROPIC MOVEMENTS, ETC. [CH. 



reference to gravitation and in part heliotropic i.e. they 

 move in response to the stimulus of light and the leaves 

 displaying their surfaces in planes more or less transverse 

 to the direction of light, and of gravitation, are said 

 to be dia-heliotropic and dia-geotropic. Mere inspection 

 shows that the leaves of a pendent or displaced branch of 

 Rubus, Beech, Elm, Lime, Philadelphus, &c, regain their 

 normal positions by curvatures of the petioles or the bases 

 of the laminae ; and more exact experimental observations 

 suggest that these curvatures are due to the unilateral 

 growth of restricted parts which have not yet ceased 

 to grow in length. This is particularly well seen in 

 leguminous plants such as Phaseolas, where the growing 

 region is the large pulvinus ; but it is also visible in other 

 cases, e.g. the Silver Fir, Taxus, &c, where the pulvinus 

 is less evident. 



While, therefore, the primary extension of the lamina 

 is due to differences in growth on different sides, these 

 secondary resumptions of position are due to the local 

 growth of certain parts of the leaf, correlated with the 

 action of gravitation or of light on the organ. 



That the position of the expanded leaf is also in part 

 a phenomenon of heliotropism, due to the directive action 

 of light, is very evident. In many cases, especially among 

 Monocotyledons, the incident light, and particularly the 

 more refrangible rays, obviously induces retardation of 

 growth of the young leaf; whereas in many Dicotyledons 

 (e.g. Tobacco) the direct contrary appears at first sight to 

 be the case, judging from the small size of the etiolated 

 leaves of plants which have been allowed to grow in the 

 dark. 



It has been shown, however, that the last result is 

 illusory, and due to a pathological phenomenon ; for if 

 the leaves of such Dicotyledons are periodically illumin- 



