IRRITABILITY. 459 



first pointed out, maintains any direction of growth which it 

 may originally have assumed, and by the aerial roots of 

 various Epiphytes. 



A reversal of the geotropic properties of an organ may 

 take place in the course of its development. Vochting has 

 found, for instance, that the peduncle of the Poppy is positively 

 geotropic whilst the flower is in the bud, but negatively geo- 

 tropic during flowering and fruiting: similarly, the peduncle of 

 Tussilago Farfara is negatively geotropic during the period 

 of flowering ; during the development of the fruit the upper 

 part of the peduncle becomes positively geotropic; and finally, 

 when the fruit is mature, the whole peduncle is negatively 

 geotropic. 



The foregoing are some examples of the phenomena of 

 geotropism as exhibited by orthotropic organs. In passing 

 now to study the action of gravity in producing them, we shall 

 subdivide the subject as we did in treating of the heliotropic 

 action of light (p. 431); we shall consider first the effect of 

 variations in the angle of deviation that is, of the angle 

 made by the long axis of an organ when in an abnormal 

 position, with its long axis when in the normal position of 

 equilibrium with regard to gravity and secondly the effect 

 of variations in the intensity of the force. It is true that 

 the force of gravity is constant ; but since, as we have seen, 

 centrifugal force induces curvature just as gravity does, we 

 can, by substituting this variable force for the constant one, 

 obtain results which are probably the same as those which 

 would be obtained could we vary the force of gravity. 



We have learned that the heliotropic effect of light is 

 determined by the angle of incidence of the rays, and we shall 

 now find that the geotropic effect of gravity exhibits a similar 

 relation to the angle of deviation. From his experiments on 

 stems and roots Sachs comes to the conclusion that the geo- 

 tropic influence of gravity is greatest when the long axis of an 

 orthotropic organ is at right angles to the vertical, and that it 

 is zero when the long axis of the organ coincides with the 

 vertical, whether the apex of the organ point upwards or down- 

 wards, or whether the organ be positively or negatively geo- 



