IRRITABILITY. 471 



found that the runners of Fragaria canadensis do not remain 

 horizontal when kept in darkness, but curve upwards, and he 

 rightly infers that they are negatively geotropic. He has 

 likewise found that the lateral branches of Conifers and di- 

 cotyledonous trees do not in all cases maintain their hori- 

 zontal direction of growth in darkness, and when this is the 

 case he accounts for it in a different way. For instance, he 

 found that the upward curvature of such branches was in 

 some cases interfered with, and in others prevented, by the 

 weight of the leaves : here the cause of the more or less com- 

 plete maintenance of the horizontal direction of growth is a 

 purely mechanical one. In other cases he observed that the 

 leafless branch (Tilia) maintained its horizontal direction of 

 growth in darkness when its morphologically superior sur- 

 face was uppermost, and that it did not do so, but curved 

 upwards, when its morphologically superior surface was 

 undermost. The branch, he says, was clearly epinastic ; 

 the maintenance of the horizontal direction of growth in the 

 first instance, was due to the exact counterbalancing of nega- 

 tive geotropism by epinasty; the upward curvature in the 

 second instance, to the cooperation of negative geotropism 

 and epinasty. In other cases (Ulmus, Corylus, Picea) he 

 found that leafless shoots, when placed in the normal hori- 

 zontal position, curved upwards, and when placed in the in- 

 verse position, downwards. These shoots were hyponastic. In 

 the former position hyponasty and negative geotropism co- 

 operated and produced the upward curvature ; in the latter 

 they acted antagonistically, but the former was sufficiently 

 strong to overcome the latter, and the downward curvature 

 resulted. In a word, the positions assumed by dorsiventral 

 shoots growing in darkness are regarded by de Vries as the 

 results of the simultaneous action of spontaneous heterauxesis 

 and negative geotropism. 



With regard now to the special peculiarity of the dia- 

 geotropism of dorsiventral organs, the maintenance of the 

 morphologically superior and inferior surfaces in their normal 

 relative positions. The torsions which Frank found to take 

 place when dorsiventral shoots were placed with their superior 



