420 ME. F. DARWIN ON THE POSITION OF 



On the Power possessed by Leaves of placing themselves at Right 

 Angles to the Direction of Incident Light.- By Francis 

 Darwd*, F.L.S. 



[Read December 16, 1880.] 



It is well known that certain organs of plants have the power of 

 growing in oblique or horizontal directions. The rhizomes of 

 many plants extend horizontally under ground, thus growing at 

 right angles to the direction of gravitation instead of parallel 

 to it, like ordinary geotropic or apogeotropic* stems and roots. 

 When the obliquely-growing organ is above ground the case 

 becomes more complex ; for here it is not only growing obliquely 

 with respect to the gravitation, but also with respect to the chief 

 portion of incident light, namely that which comes from the 

 zenith. 



A seedling radish growing in open ground may illustrate the 

 case. The hypocotyl grows vertically upwards, being both apo- 

 geotropic and heliotropic, and therefore growing parallel to the 

 incident light which falls on it from above, and also to the line of 

 gravitation. But the cotyledons place themselves in a nearly 

 accurately horizontal position, at right angles both to the line of 

 gravitation and to the light. It will be found, however, that it 

 is the direction of the light, and not that of gravitation, which 

 determines the position of the cotyledons ; for if the seedling be 

 placed (with due precautions) before an oblique lateral light its 

 cotyledons adjust themselves so that their surfaces are once 

 more at right angles to the direction of the light. 



The following investigation is an attempt to arrive at a clearer 

 conception of the nature of the power which leaves possess of 

 thus placing themselves at right angles to the direction of in- 

 cident light. 



It will first be necessary to give, as an introduction to my own 

 work, a short account of the state of knowledge on the subject. 



The two chief theories which I shall have to consider are 

 those of A. B. Frank and of De Vries. Frank t supposes that, 



* Geotropic is used to mean positively geotropic, apogeotropic to me 011 

 negatively geotropic, in accordance with the terminology used in ' The Po*er 

 of Movement in Plants/ by C. Darwin, assisted by P. Darwin, 1880, p. 5. 



t ' Die natiirliche wagerecbte Richtung von Pflanzentheilen :' Leipzig, 1870- 



