464 The Physiology of Plants BOOK in 



account of the number of both authors and papers it is 

 impossible to discuss them here, but references will be 

 made to the more important ones later. 



HELIOTROPISM 



The behaviour of different parts of plants with radial 

 symmetry, when exposed to differences of illumination on 

 two opposite sides, was observed in comparatively early 

 times, and attempts to explain it were put forward by 

 Dutrochet. The fact that roots and stems curve in opposite 

 directions, the former away from, the latter towards, the 

 direction of greatest intensity, was also known. The terms 

 Positive and negative heliotropism were first applied to these 

 curvatures by Hofmeister in 1863 ; they were called helio- 

 tropism and apheliotropism by Darwin in 1880. 



In 1870 Frank described a third movement effected under 

 similar conditions, the assumption by organs of dorsiventral 

 symmetry of a position across the incident rays, and gave 

 it the name of transverse heliotropism, a term later replaced 

 by diaheliotropism by Darwin in 1880. Frank's view was 

 not accepted at once, De Vries, Sachs, and other writers 

 suggesting that the position is not taken up in consequence 

 of the incidence of the light, but is the result of the working 

 of other forces, such as the weight of the organ acting in 

 opposition to positive heliotropism. 



The subject of transverse heliotropism was the subject 

 of considerable investigation between 1870 and 1880. 

 Frank showed in 1870 that it is exhibited not only by 

 leaves which have an anatomically dorsiventral symmetry, 

 but also by certain stems, which, though anatomically 

 radial, are physiologically bilaterally symmetrical. Shoots 

 of Lysimachia nummularia, Polygonum aviculare, and several 

 other plants which grow erect in darkness, run along the 

 ground when light is admitted to them. In 1873 he showed 

 that branches of the thallus of Marchantia become narrow 



