219 
their plumes to ascend, by some external cause, and not by any power 
inherent in vegetable life: and I see little reason to doubt that gravitation 
is the principal, if not the only agent employed, in this case, by nature. I 
shall therefore endeavour to point out the means by which I conceive the 
same agent may produce effects so diametrically opposite to each other. 
The radicle of a germinating seed (as many naturalists have observed) 
is increased in length only by new parts successively added to its apex or 
point, and not at all by any general extension of parts already formed; and 
the new matter which is thus successively added, unquestionably descends 
in a fluid state from the cotyledons. * On this fluid, and on the vegetable 
fibres and vessels whilst soft and flexible, and whilst the matter which com¬ 
poses them is changing from a fluid to a solid state, gravitation, I conceive, 
would operate sufficiently to give an inclination downwards to the point of 
the radicle; and as the radicle has been proved to be obedient to centrifugal 
force, it can scarcely be contended that its direction would remain unin¬ 
fluenced by gravitation. 
I have stated that the radicle is increased in length only by parts suc¬ 
cessively added to its point: the plume, on the contrary, elongates by a 
general extension of its parts previously organized; and its vessels and fibres 
appear to extend themselves in proportion to the quantity of nutriment they 
receive. If the motion and consequent distribution of the true sap be in¬ 
fluenced by gravitation, it follows, that when the plume, at its first emis¬ 
sion, or subsequently, deviates from a perpendicular direction, the sap must 
accumulate on its under side: and I have found in a great variety of expe¬ 
riments on the seeds of the horse chesnut, the bean, and other plants, when 
vegetating at rest, that the vessels and fibres on the under side of the 
plume invariably elongate much more rapidly than those on its upper side; 
and thence it follows that the point of the plume must always turn up¬ 
wards. And it has been proved that a similar increase of growth takes 
place on the external side of the plume when the sap is impelled there by 
centrifugal force, as it is attracted by gravitation to its under side, when 
the seed germinates at rest. 
This increased elongation of the fibres and vessels of the under side is 
not confined to the plumes, nor even to the annual shoots of trees, but 
* See Philosophical Transactions of 1805. 
