376 DIRECTION OF THE GROWTH OF ROOTS, 



Explanation of When the seeds of the bean, in the experiments I hqve 



the growth of (described, were placed to veeetate beneath the mould of an 

 the beans, p. 7 t r^ . 



372. inverted pot, a sufficient quantity of moisture was afforded 



by the mould, to occasion the protrusion of the radicles : 

 but as soon as the under points of these had penetfaied 

 r through the seed-coats, their surfaces were necessarily ex- 



posed to dry air, and were consequently rendered rigid and 

 inexpansible; while their upper surfaces, being in contact 

 with the moist mould, remained soft and expansible. If 

 both the upper and lower surfaces of the radicles, at their 

 points, had been equally well supplied with moisture, gra- 

 vitation would have attracted the sap to the lower sides, 

 where new matter would have been added; and the radicles 

 would have extended perpendicularly downward, as in for- 

 xner experiments; but the influence of gravitation was, to a 

 great extent, counteracted by the effects of drought upon 

 the lower sides of the radicles, nearly as it was counteracted 

 by centrifugal force, when made to act horizontally *. 



As soon as the radicles had acquired sufficient age and 

 maturity, efforts were made by them to emit fibrous roots; 

 when want of proper moisture on the lower sides prevented 

 their being protruded, in any other direction, except up- 

 wards. In this direction therefore they were alone emitted, 

 (as I was confident that they would before I began the ex- 

 periment) and having found proper food and mpisture in the 

 pots, they extended themselves upward through more than 

 half the mould, which these contained. 

 The experiment "^^'^ experiment was repeated, and water was so con- 

 repeated with stantly and abundantly given, that every part of the radicles 

 additional ^^^ kept equally Avet; and they then became perfectly 



obedient to gravitation, without being at all influenced by 

 the mould above them. 



In other experiments, pieces of alum and of the sul- 

 Sulphates of , ^ . , , , „ ,. 



alum, iron, and phates of jTon and copper were placed at small distances 



copp^'^' d'^ "°^ perpendicularly beneath the radicles of germinating seeds, 



growth of roots of different species, to afford an opportunity of observing, 



but by actual whether any efforts would be made by them to avoid 



poisons; but they did not appear to be at all influenced, 



* Phil. Trans. 1806, p. 6: or Journal, vol. xiv, p. 411". 



except 



moisture. 



