IRRITABILITY. 461 



completely to bring about the normal vertically downward 

 direction of the apex. Doubtless the geotropic irritability of 

 the roots growing in earth was greater than that of those 

 growing in moist air. But the latter retained considerable geo- 

 tropic irritability nevertheless. This is shewn by the fact that 

 when, after having curved as a consequence of their reversed 

 position, they were so moved that the apices again pointed 

 vertically upwards, they again curved downwards to a greater 

 or a less extent. It is shewn further, and perhaps more 

 strikingly by another experiment. Seedlings were placed 

 with their radicles pointing radially inwards on a wheel rotating 

 with such a velocity that the acceleration of the centrifugal 

 force was about 50 * (g being the acceleration due to gravity), 

 and care was taken to ensure a moist atmosphere. Under 

 these circumstances they naturally tended to curve outwards, 

 as in Knight's experiment already described (p. 456), for they 

 were in an inverted position as regards the direction of the 

 centrifugal force ; and they did so to a much greater extent 

 than in the previous experiments under the action of gravity. 

 In no case was the ultimate angle of deviation from the normal 

 position (radially outwards) greater than 45. These roots, 

 then, possessed geotropic irritability, though in a less degree 

 than roots grown under normal conditions. This, far from 

 being an objection, is a support to Elfving's view. These 

 feebly geotropic roots afforded a better means of estimating 

 the geotropic effect of gravity at different angles than highly 

 irritable roots. 



A further objection to Elfving's conclusion might be based 

 on Sachs' observation that geotropic curvature is more rapidly 

 produced when an organ is placed horizontally than when it 

 is in any other position. Elfving has anticipated this objection, 

 and urges that this may be due to the fact, mentioned in 

 a previous lecture (p. 409), that when an organ is placed in 

 the inverted position its growth is retarded by the action of 

 gravity, and consequently its curvature is slowly produced. 



Elfving's experiments are obviously incomplete ; they 

 require to be extended to shoots and to other organs. But 

 they seem to point to this general conclusion, that the geotropic 



