470 The Physiology of Plants BOOK in 



and to some extent studied by earlier writers. The ten- 

 dency of roots to grow downwards, and of stems to grow 

 upwards were named by him -positive and negative geo- 

 tropism respectively. He added to our knowledge of the 

 directive influence of gravitation by describing the be- 

 haviour of certain radial organs, such as rhizomes, which 

 grow parallel to the surface of the soil, and to this tendency 

 he applied the term transverse geotropism, which was re- 

 placed by the name diageotropism by C. Darwin in 1880. 

 Darwin's terminology, geotropism and apogeotropism, for 

 positive and negative geotropism are now in common use. 

 The writers who examined and discussed the phenomena 

 of heliotropism also concerned themselves with these curva- 

 tures and demonstrated the co-existence of the two kinds 

 in most plants, pointing out as well their similarity. 

 Allusion may be made especially to the researches of 

 Wiesner and Darwin in this direction. The latter writer 

 added greatly to our knowledge of this form of irritability. 

 Elfving and Stahl may also be mentioned in this connexion. 



Frank attributed to the possession of transverse geo- 

 tropism the plagiotropic position assumed by dorsiventral 

 organs such as leaves, but his views met with considerable 

 opposition from De Vries, who claimed that the assump- 

 tion of their position should preferably be regarded as due 

 to a co-operation of many causes, negative geotropism 

 acting concurrently with light, the latter not only causing 

 heliotropic curvature but epinastic growth. 



The explanations that were advanced to explain the 

 curvatures underwent a change about 1868 and 1870, when 

 Frank revived the theory of response to stimulation which 

 had been advanced by Dutrochet in 1824 an d recognized 

 by Treviranus in 1838, but which had fallen into the 

 background. The views which had supplanted it were 

 based upon purely mechanical considerations. Knight had 

 held still earlier, in 1806, that the curvatures are due to 



