21 6 Mr. Knight on the Causes which irjluence 
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 de- 
scribed, 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 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, 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 hori- 
zontally.* 
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 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 contained. 
* Phil. Trans. 1S06, p. 6. 
