IRRITABILITY. 477 



axis in such a position with regard to the direction of the 

 incident rays that each side of the organ under observation 

 receives an equal amount of illumination, when heliotropic 

 curvature is an impossibility. The influence of gravity can be 

 eliminated by rotation on a clinostat. In experiments per- 

 formed under these conditions, Sachs found the tendency 

 towards perpendicular growth to exhibit itself in an unmistak- 

 able manner, in the growth of the sporangiferous hyphse 

 of Moulds and of the shoots of seedlings of various kinds. 

 The radicles of the seedlings did not, in most cases, grow 

 straight inwards into the piece of turf, as the accompanying 

 figure shews. This is due to the disturbing influence of 

 another directive agent, the moistness of the substratum, an 

 influence which we shall shortly proceed to consider. 



These facts clearly prove that the substratum exercises 

 a directive influence upon the growth of organs developed 

 upon it. This influence is sufficiently powerful, as the above 

 figure shews, to induce heterauxesis and thus to give rise to 

 curvatures. 



Moisture. The fact that roots, when brought into the 

 neighbourhood of moist surfaces, curve towards them, appears 

 to have been long known to physiologists. Bonnet mentions 

 it, but Knight seems to have been the first to make it the 

 subject of experiment, and in this he has been followed by 

 Johnson, Duchartre, Sachs, Darwin, Wiesner, and Molisch. 

 The mode of experiment is very much the same in all cases. 

 Seeds are sown in damp moss or sawdust contained in a 

 vessel, suspended vertically or obliquely, the bottom of which 

 is perforated with holes large enough to allow the roots of the 

 seedlings to pass through them. In consequence of their 

 positive geotropism, the primary roots of the seedlings grow 

 downwards till they pass out into the air through the holes in 

 the bottom of the vessel. Then their direction of growth 

 alters. They no longer grow vertically downwards, but curve 

 upwards so as to apply themselves to the moist surface : the 

 influence of gravity has been overcome by the action of 

 another stimulus which calls forth from the roots a more 

 powerful response. These observations do not, however, 



