850 MECHANICS OF GROWTH, 
elevate itself, and in consequence of the persistent effect of gravitation and of the 
curvature of the middle and basal portions, it may pass beyond the vertical ; finally it 
stands erect. The middle portion remains, sharply curved; the basal portion is but 
slightly curved, for its growth ceases before it is possible for it, in consequence of its 
unfavourable position, to undergo any great curvature. 
Groivtb and CurDature ivithout Absorption of Water. If shoots consisting of a growing 
part and of a part which has ceased to grow be cut off and placed erect (the apex being 
uppermost) in a dry glass cylinder which is then closed in order to prevent excessive 
evaporation, they continue to grow for a considerable time without any absorption of 
water, and at the same time they lose a portion of their water by evaporation into the 
closed spaced It might be assumed that the water requisite for the elongation of the 
growing portion was derived from the part which had ceased to grow. If, however, the 
growing region alone be cut off and the terminal bud removed, and then marks be made 
on the shoot, it becomes evident that all the segments of the shoot grow without absorb- 
ing any water. The elongation is certainly less than usual, but it is distinct. 
If shoots consisting of a part which is growing and of a part which has ceased to grow 
be cut off and placed horizontally in a closed space and protected from excessive evapora- 
tion, a curvature occurs in the growing region which may result in the elevation of the 
apex into an erect position. In this case the water which is necessary for the more 
rapid growth of the under side of the shoot might be absorbed from the posterior 
fully-developed parts. If, however, only the growing region of the shoot be cut off, or 
a single internode, the upward curvature will still take place, and, in this case, throughout 
the whole piece. Accompanying this process we have (i) a loss of weight due to the 
evaporation of water into the unsaturated atmosphere, (2) an elongation of the convex 
lower surface, corresponding to the upward curvature, and (3) no elongation, or a very 
slight one, but more generally a contraction, of the upper concave side. 
Cur'vature of split shoots. If the growing region of a shoot is split into two symme- 
trical halves which remain connected posteriorly by a portion of the shoot which has 
completed its growth, they will curve concavely outwards in consequence of the 
tension of their tissues. If, whilst thus curved, the epidermis of the two concave 
surfaces and the two convex cut-surfaces of the pith be measured, and if then the 
shoot be placed in such a way that the epidermis of one surface is directed downwards 
and that of the other upwards, each half will exhibit geotropism. The growth of the 
pith of the upper half will be accelerated, whereas that of the cortex of the same half 
will be retarded or the cortex may even become shorter; in the lower half, the growth 
of the pith will be retarded and that of the cortex accelerated. The following were the 
increments of growth in 24 hours observed in Sylphium connatum : — 
TT , J. , 1 ir ( epidermis (above) . . . - I'o m.m. 
Upper longitudmal half \ r f vu /u 1 \ 
^ ( surface of pith (below) . . +107 „ 
Lower longitudinal half " (^''^^^ • " + " 
° ( epidermis (below) . . . + 2*0 „ 
The same takes place, only in a more marked manner, in the case of the haulms of 
Grasses, which are more adapted for observations of this kind, for, when they are split, 
the two halves do not curve outwards. 
If a longitudinal slice be taken from the middle of the stem of some Dicotyledon 
which is not hollow but which has a thick pith {e. g. Senecio Doria), by paring away the 
wood symmetrically on each side, it is possible to place it horizontally in two ways, 
{a) that in which the cut surfaces are vertical, and {b) that in which the cut surfaces are 
^ It is to be remembered that many shoots, such as those of Fritillaria imperialis, are much 
disturbed in their growth if the apex be cut off ; and they hardly grow at all if they are cut off at the 
base. As a consequence the curvature of such shoots is very slight or entirely absent. 
