162 ON THE DIRECTION 



in a wet soil ; and on this account the roots of the one will appear to 

 have sought, and those of the other to have avoided, the contiguous 

 water ; though both, in the first period of their growth, pointed their 

 roots alike in every direction. 



When the seeds of the bean, in the experiment I have described, were 

 placed to vegetate beneath the mould of an 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 penetrated 

 through the seed-coats, their surfaces were necessarily exposed to dry air, 

 and were consequently rendered rigid and inexpansible ; whilst their 

 upper surfaces, being in contact with 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, gravitation would 

 have attracted the sap to the lower sides, where new matter would have 

 been added; and the radicles would have extended perpendicularly 

 downwards, as in former 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 upwards. In that direction therefore they were alone emitted, 

 (as I was confident that they would before I began the experiment) and 

 having found proper food and moisture in the pots, they extended 

 themselves upwards through more than half the mould, which these 

 contained. 



This experiment was repeated, and water was so constantly and abun- 

 dantly given, that every part of the radicles was kept equally wet ; 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 sulphates of iron and 

 copper were placed at small distances perpendicularly beneath the radicles 

 of germinating seeds, of different species, to afford an opportunity of 

 observing whether any efforts would be made by them to avoid poisons ; 

 but they did not appear to be at all influenced, except by actual contact 

 of the injurious substances. The growth of their fibrous lateral roots 

 was, however, obviously accelerated, when their points approached any 

 considerable quantity of decomposing vegetable or animal matter : and 



* Above, p. 125 



