500 LECTURE XIX. 



growing erect no longer curved so as to become horizontal, 

 whereas those on the clinostat became horizontal in eight 

 hours. 



In the following table are given Wiesner's determinations of the 

 angles which the secants of the curvatures of the cress-shoots which 

 were not rotating made with the vertical at different distances from the 

 source of light. 



Distance from the flame. Angle with the vertical. 



0-25 metres 30 



o-3o 35 



075 55 



1-25 70 



2*50 80 (optimum) 



3-00 65 



375 35 



The foregoing results clearly shew that the relation between 

 the geotropic and the heliotropic irritability is by no means the 

 same in the shoots of all plants, and this affords an explana- 

 tion of the different directions of growth assumed by the 

 shoots of different plants growing under the same con- 

 ditions. 



Turning now to primary roots, when they are growing in 

 soil, the external directive influences which may act upon them 

 are those of gravity and of an unequally distributed moisture 

 (p. 477). They grow vertically downwards, in consequence of 

 their positive geotropism, when the soil around them is uni- 

 formly moist, but, inasmuch as their hydrotropic irritability is 

 greater than their geotropic, they will, when the soil is not 

 uniformly moist, curve out of the vertical and grow towards 

 the damper area. 



The relation between the geotropic and heliotropic irritabi- 

 lity is of interest only as a matter of experiment in the case of 

 earth-roots, but it is a matter of biological importance in the 

 case of aerial roots. It has been already pointed out (p. 436) that 

 the heliotropic irritability of the former is slight, so slight in most 

 cases that it can only be detected, if at all, by eliminating the 

 action of gravity by means of the clinostat. The heliotropic 

 irritability of aerial roots is, on the contrary, frequently well- 



