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POPULAR SCIENCE REVIEW. 
that increased growth, first on one side and then on another, is 
a secondary effect ; and that the increased tnrgescence of the 
cells, together with the extensibility of their walls, is the 
primary cause of the movement of cir cumnutation. * 
It will be advisable to treat of the different organs in a 
definite order, so I will proceed to describe the motions of 
radicles, cotyledons, hypocotyls, and epicotyls of germinating 
seeds ; then will follow those of the stems and leaves of fully 
developed plants ; and finally, the movements effected by the 
different organs of the floral region. 
Radicles. — The tip of the radicle, as soon as it protrudes 
from the seed-coat, begins to circumnutate, and the whole 
growing part continues to do so, probably for as long as 
growth is maintained ; it presumably is aided by this motion 
in being guided along lines of least resistance. As soon as the 
radicle has protruded, geotropism at once acts upon it. If this 
force be identical with gravitation, it is gravitation acting in a 
peculiar way ; for although the radicles of germinating seeds, 
whether in England or at the Antipodes, point to the centre of 
the earth, yet the influence affects the tip only, for a length 
of no more than the *02 to *03 of an inch, at least, in the 
cabbage. This minute part, however, at once communicates 
some influence to a point further back, where the radicle bends 
downwards in response to it ; neither geotropism nor cir cum- 
nutation can enable the radicle to penetrate the soil. This is 
effected by the force due to growth both longitudinally and 
transversely. By means of ingenious contrivances, seeds were 
grown under resistance ; and Mr. Darwin found that transverse 
growth exerted a force, after six days, of more than 8 lbs., 
and vertical growth, after twenty-four hours, of at least a 
\ lb. The purchase for the due effect of these forces is gained 
partly by the seed being below the soil, aided by the root- 
hairs, which, in consequence of the cellulose passing into a 
gum-like substance, fix the radicle to the particles of the 
soil. The growing part, therefore, acts like a wedge of wood, 
which whilst slowly driven into a crevice, continually expands 
at the same time by the absorption of water ; and a wedge thus 
acting, Mr. Darwin observes, will split even a mass of rock. 
Another important property of radicles is their extreme 
sensitiveness to irritants, such as mechanical obstructions, 
caustic, and injuries by being cut, all of which induce the 
radicle to bend away from the exciting cause, as well as to the 
presence of moisture, which, on the contrary, induces the 
radicle to bend towards it. Any solid body which gives rise to 
* P. 2, 3. I have not here or elsewhere added inverted commas, for 
throughout this paper I have largely interwoven Mr. Darwin’s sentences 
with my own. 
