7 86 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. 



(</) 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 cur'vature 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 b^nt, 

 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 Senecioy flower-stalks 

 of Digitalis J 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 diff"erent 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 ripst, 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 Wiirzburg, Bd. I, 1874. 



^ The phenomenon here described was first observed and studied by Hofmeister (Jahrb. fiir 

 wissensch. Bot. vol. II, i860); and a few important corrections of his description were given by 

 Prillieux (Ann. des Sci. Nat. ser. 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. 



