IRRITABILITY. 473 



In summing up the evidence for and against the assump- 

 tion of diageotropism, we find that it is unassailed as regards 

 radial organs, and in fact, with the case of the rhizomes 

 observed by Elfving (p. 462) in view, it seems to be unassail- 

 able. But the case for the diageotropism of dorsiventral 

 organs has not been as clearly made out as that for their 

 diaheliotropism (p. 449) ; before the question can be regarded 

 as finally settled more experimental evidence must be forth- 

 coming. 



A full discussion of the details of the mechanism by which 

 geotropic curvature is effected will be given in a subsequent 

 lecture when the general question of curvature will be gone 

 into. It need only be urged here, as it was previously 

 with regard to heliotropism and it is more obviously true in 

 the case of geotropism that curvature is not the expression 

 of the directive action of the force on the cells or cell-walls, 

 retarding the growth of one side of the organ and accelerating 

 that of the other, but that it is the response of the organ 

 as a whole to the action of the force, whereby it is stimulated 

 to take up a definite position with regard to the direction in 

 which the force acts. 



Current of Water. Jonsson has observed that the direction 

 of growth of plant-organs is affected when they are exposed 

 to the influence of a current of water ; he designates the in- 

 duced phenomena by the term Rheotropism. Organs grown 

 under this condition, place themselves so that their long axes 

 lie in the direction of the current. The hyphae of Moulds (Phy- 

 comyces, Mucor) took up such a position that their direction 

 of growth coincided with the direction of the current, they 

 were, as Jonsson puts it, positively rheotropic. The hyphae 

 of Botrytis cinerea^ however, took up such a position that their 

 direction of growth was opposed to that of the current ; they 

 proved themselves to be negatively rheotropic. The radicles 

 of Maize-seedlings likewise proved themselves to be negatively 

 rheotropic, as did also those of Rye- and Wheat-seedlings. 



It appears, then, that the force of a current of water 

 exercises a directive influence, that it, in fact, induces heter- 

 auxesis, and so give rise to curvatures of plant-organs. 



