IRRITABILITY. 409 



GRAVITATION. 



In view of the influence which different temperatures and 

 different intensities of light exercise upon the rate of growth 

 in length, the question naturally suggests itself, inasmuch as 

 growing organs respond by curvatures to alterations in their 

 relation to the line of action of gravity, whether or not their 

 rate of growth would be affected by reversing their normal 

 relations to the line of action of gravity, or by neutralising 

 the action of gravity, or, finally, by exposing them to the 

 action of a centrifugal force, which has the same action upon 

 them as gravity, either greater or less than that of gravity. 

 Elfving has endeavoured to give an answer to this by his 

 experiments on organs which normally grow vertically, either 

 upwards or downwards. He found, in the first place, that the 

 sporangiferous hyphae of Phycomyces nitens grow rather less 

 rapidly in the inverted than in the normal erect position. 

 He concludes that this retardation is due to the alteration of 

 the relation of the axis of the organ to the line of action of 

 gravity, and he considers it to be probable that the effect 

 would be the same in the case of all organs which normally 

 grow upwards. This conclusion is supported by the observa- 

 tions of Vochting, who has noted that the pendent branches 

 of so-called " weeping trees " grow in length less rapidly than 

 their erect branches. This point cannot, however, be re- 

 garded as having been fully investigated, for the corresponding 

 observations have not yet been made upon organs which 

 normally grow downwards, but we may perhaps compare the 

 action of gravity upon the rate of growth of an organ in its 

 normal condition to the effect of the optimum temperature 

 upon the rate of growth, and the effect of a reversal of the 

 normal relation of the organ to the line of action of gravity 

 to the effect of a temperature either higher or lower than 

 the optimum. In the second place, Elfving has found, by 

 further experiments with Phycomyces, that the rate of growth 

 of the hyphse is the same whether they grow in their normal 

 position, or whether the action of gravity upon them is 

 neutralised by causing them to rotate slowly round a hori- 



