ACTION OF GRAVITATION ON GROWTH IN LENGTH. 
^ folds are found upon the concave under surface in consequence of compression, just 
as in the nodes of Grasses. When the root is curving geotropically, all the cells 
within the curving portion usually grow, but their growth is slower the nearer they 
are to the lower surface which is becoming concave. In passing from the convex 
surface, where the cells are fully developed and contain much sap, to the concave surface, 
where the cells present the appearance of young undeveloped cells containing much 
protoplasm, all intermediate forms may be met with. Since, therefore, the development 
of the cells of the under side is very considerably impeded, it is possible for those 
of the upper side to undergo a more or less excessive elongation. Some observations, 
as yet incomplete, seem to indicate that the retardation of 
the growth in length of the under side is accompanied by a 
more vigorous growth of the cells in a radial direction, and 
the acceleration of that of the upper surface by a less vigorous 
radial growth. 
If thick primary roots be split and be treated in the manner 
described with reference to stems, the same phenomena 
(though in exactly the opposite direction) are generally pro- 
duced : this shows that geotropism is not merely a property 
of the root as a whole, but also of each of its constituent 
parts. These observations are, however, very difficult to carry 
out. A persistent effect of the commencing geotropic action, 
which was so well-marked in the case of stems, is stated 
to occur in roots also by Ciesielski and Frank. I have not yet 
succeeded in detecting it, but I will not reject the fact, for 
other methods will perhaps afford more satisfactory results. 
4. The chief result of the observations which I have made 
hitherto is, I believe, this, that the phenomena of upward 
geotropic curvature are essentially the same, though taking 
place in the opposite direction, as those of downward geotropic 
curvature, and that therefore the mechanical explanation of the 
one will include that of the other. This necessarily implies 
the incorrectness of the older explanations offered by Knight 
and by Hofmeister. 
Knight, the discoverer of the fact that it is gravitation 
which induces geotropic curvature, believed the upward cur- 
vature of the stem to be due to an accumulation of nutrient 
materials towards the lower surface, which would induce 
more vigorous growth. Hofmeister, who regarded the state 
. of tension of the tissues to be the most important factor in 
producing the curvatures of parts of plants, considered that 
the action of gravitation in producing an upward curvature 
was to increase the extensibility of the passively stretched 
tissues of the lower side. I pointed out that the growth of 
the under surface of an organ capable of curving upwards 
was accelerated, and that of the upper surface retarded : I 
did not at the time express an opinion as to whether these modifications of growth 
were due to an altered distribution of plastic material or to a change in the extensibility 
of the passive layers of tissue. 
Knight explained the downward curvature of primary roots in a somewhat obscure 
. manner by referring it to the softness and flexibility of the growing apex, a view 
which was adopted by Hofmeister in a more precise and logically complete form, 
and one which I for a time accepted. It was assumed that the tissue of the growing 
root was comparable to soft dough, and that the unsupported end tended to curve 
downwards under the influence of its own weight. I considered that the weight of 
A 
Fig. 484.— The growing and 
curving end of the primary root of 
I'tcia Faba placed horizontally in 
loose earth behind a thin lamina 
of talc. The root was marked out 
into five portions, each two milli- 
metres in length, beginning from 
tVi& pu7tct2im vegetationis. A tri- 
angular index of paper was fixed 
to the talc so as to correspond to 
the mark o on the root ; by means 
of this the change of position of 
the marks could be detected. A , 
at the commencement of the ex- 
periment, B after one hour, C at 
the end of two hours, D after seven 
hours, E after twenty-three hours. 
