the Direction of the Groiulh of Roots. 425 



both, in the first period of their growth, pointed their root? 

 alike in every direction. 



When the seeds of the bean, in the experiment T have 

 described, were pbcid 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 ex- 

 posed to drv air, and were consequently rendered nuid and 

 inexpansible; whilst their upper surfaces, hemg 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 tf) the lower sides, 

 where new matter would have be>.n added ; and tlie radicles 

 would have extended perpendicularly downwards, as in 

 former experiments : but the nifluence of gravitat on was, 

 to a great extent, counteracted by the effects oi droui:ht upon 

 the lower sides of the radicles, nearly as it was counteract- 

 ed by centrifugal force, uhen inade to act honzontally*. 



As soon as the radicles hud acquired sufficient age and 

 maturity, efforts were made by ihcm 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 that 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 moisture in 

 the pots, they extended themselves upwards through more 

 than half the mould which these contamed. 



This experiment was repealed, and water was so con- 

 stantly and abundantly lilvcn, that every part of the radicles 

 was kept equally \\et j and they then became perlectlv obe- 

 dient to gravitation, wiihoul 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 perpen- 

 dicularly beneath the radicles of germinating seeds, of dif- 

 ferent species, to afford an opi>ortunity of observina; whe- 

 ther any efforts would be made by them to avoid poisons; 

 but they did not appear to be at all influenced, exce|)t by 

 actual contact of the injuri(yus substances. The growth of 

 their fibrous lateral roots was, however, obviouslv accele- 

 rated, when their points approached any considerable quan- 

 tity ot decomposing vegetable or animal matter : and when 



• Phil. Trans. IfiOC, p. C. 



tb« 



