786 
MECHANICS OF GROWTH, 
These observations show, as was to be expected, that the permanent curvature 
of an internode is connected with a permanent contraction of the concave and 
lengthening of the convex side. 
(d) As to the Distribution of Extensibility in growing shoots, the observations of 
de Vries^ lead to this result, that in growing strongly turgescent shoots a maximum 
of extensibility and of flexibility exists immediately below the terminal bud. From 
this point they diminish as the distance from it increases, and therefore also with 
the age of the parts. This statement holds good independently of the age of the 
growing shoots. 
{i) Sudden cuwature of grooving shoots from a blo'W or concussion. If upright growing 
shoots^ are suddenly and violently struck below at a point where growth has ceased, 
the curvature thus caused advances upwards in the form of a wave, so that immediately 
after the blow has been given to the lower part the apex of the shoot is strongly bent, 
the concavity of the curvature lying on the side from which the blow was received. 
The elasticity of the bent part causes the apex to spring back immediately; but when, 
as we have seen, the elasticity is very imperfect, the shoot retains a part of its curvature. 
As soon as the shoot has come to rest after some oscillations, it may be observed that 
below the apex, where the shoot is most flexible to an ordinary passive curvature, a 
permanent curvature is established, the apex bending over, and always on that side 
fro n which the blow was received. In many cases this phenomenon is produced by 
a single blow from a stick, as e.g. in Fagopyrum, Lythrum, and Senecio, flower-stalks 
of Digitalis, Cimicifuga, Aconitum, &c. ; in more rigid stems which are less flexible and 
more elastic at the corresponding part, the bending over of the apex does not take 
place till after three or four or even from twenty to fifty blows have been given to the 
lower woody part ; the amount of curvature also varies in different plants. If shoots 
are cut off" low down so that a woody piece which has ceased to grow can be 
taken in the hand, and the shoot made to oscillate rapidly backwards and forwards, 
it assumes, when it comes to rest, a distinct curvature below the apex in the region 
of greatest flexibility. The plane of curvature coincides with that in which the 
oscillations take place, and the apex may bend to either side; but the permanent 
curvature will always be concave on the side on which the oscillations were 
strongest. If finally a rooting shoot or one firmly held in the hand receives 
repeated lateral blows at its summit, that is, above the most flexible part, a perma- 
nent curvature is produced in this region, but it is in this case convex to the side 
from which the blows came. 
In all the cases which I have described the position of the permanent curvature 
is the same as that of the strongest curvature, even if acquired only momentarily 
by the shoot. The appearance is precisely the same as if the shoot were taken in 
the hand and then strongly bent once, or as if it were repeatedly bent backwards 
and forwards, but more strongly in one direction. Mere concussions which produce 
no strong flexion of the shoot cause no permanent curvature; if shoots are 
enclosed in glass tubes and violent impulses repeatedly imparted to them by jerking 
the tubes upwards or swinging them from side to side no change is visible when 
the shoots are removed from the tube. 
If the part of a shoot susceptible of curvature is marked with ink in equidistant 
divisions, and is then made to oscillate by blows below this part, the convex side 
of the permanent curvature is found to have become longer, the concave side 
^ Ueber die Dehnbarkeit wachsender Sprosse, Arb, des Bot. Inst, in Würzburg, Bd. I, 1874. 
^ The phenomenon here described was first observed and studied by Hofmeister (Jahrb. für 
wissensch. Bot. vol. II, i860); and a few important corrections of his description were given by 
Prillieux (Ann. des Sei. Nat. s^r. 5, vol. IX). The statements here made, which confirm the previous 
observations in all essential points, while differing from them in a few others, are entirely based on 
my own observations. 
