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. 



Growth and Curvature without Absorption of Heater. 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 space ^. 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. 



Curvature 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 : — 



epidermis (above) . . . - I'o m.m. 



+ 107 „ 



TT 1 -i. J- 1 t- ir ( epidermis (above) 



Upper longitudinal half \ r r --u /u i \ 

 ^^ ° ( surface of pith (below) 



Lower longitudinal half P"':f''°fP'"?(='J"'™) " " + '■" " 

 ( 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 {&) 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. 



