170 Mr. Knight on the Origin and Formation of Roots. 
point, and never, like the stem or branch, by the extension 
of parts previously organized ; and I have endeavoured to 
shew, in a former memoir, that owing to this difference in the 
mode of the growth of the root and lengthened plumule of 
germinating seeds, the one must ever be obedient to gravita- 
tion, and point towards the centre of the earth, whilst the 
other must take the opposite direction.* But the radicle of 
germinating seeds elongates by the extension of parts pre- 
viously organised, and in a great number of cases, which must 
be familiar to every person’s observation, raises the cotyle- 
dons out of the mould in which the seed is placed to vegetate. 
The mode of growth of the radicle is therefore similar to that 
of the substance which occupies the spaces between the buds 
near the point of the succulent annual shoot, and totally dif- 
ferent from that of the proper root of the plant, which I con- 
ceive to come first into existence during the germination of 
the seed, and to spring from the point of what is called the 
radicle. At this period, neither the radicle nor cotyledons 
contain any alburnum ; and therefore the first root cannot 
originate from that substance ; but the cortical vessels are then 
filled with sap, and apparently in full action, and through 
these the sap appears to descend which gives existence to the 
true root. 
When first emitted, the root consists only of a cellular sub- 
stance, similar to that of the bark of other parts of the future 
tree, and within this the cortical vessels are subsequently ge- 
nerated in a circle, inclosing within it a small portion of the 
cellular substance, which forms the pith or medulla of the 
root. The cortical vessels soon enter on their office of gene- 
* Phil. Trans. 1806. 
