MOVEMENTS OF GROWTH. 363 



themselves after they have been stretched, they shorten again 

 more or less ; their tissue is therefore elastic. Since, however, 

 they do not completely resume their original length, but after 

 vigorous stretching remain permanently longer, they must be 

 considered incompletely elastic. 



Perfectly fresh, straight internodes of Vitis or Aristolochia, 

 about 6 mm. thick, are bent with both hands over a card, marked 

 with concentric circles, till the axis of the stem coincides with one 

 of the circles. The known radius of this circle we note as the 

 radius of curvature of the bent internode. If we leave the stem 

 to itself, it does not become straight again, but remains somewhat 

 considerably bent ; and we may again easily determine its radius 

 of curvature. Growing plant structures are therefore flexible. 

 They have elasticity of flexion, but the elasticity is not perfect. 



If we select a straight, actively elongating shoot, with a stick 

 give it a blow or several blows in succession at the lower end, 

 where the growth in length is already completed, the curvature 

 produced in the region struck travels in the form of a wave right 

 tip to the free end. This consequently appears bent, and the 

 concavity always lies on the side from which the blow came. 

 This curvature, resulting from a blow or concussion, which I 

 observed especially beautifully in experiments with shoots of 

 Vitis and shoots of Lonicera tatarica, owes its origin, as is more 

 fully explained in my Lehrbuch tier Pflanzenphysiologie, to the 

 flexibility and incomplete elasticity of the structures. 



1 See Sachs, Textbook of Botany. 



147. Relations between Turgidity Growth and Extensibility 



in Plants. 



For our purpose it is, first of all, indispensable to determine the 

 relative rate of growth in the successive partial zones of a plant 

 structure. We experiment with young flower-stalks of Butomus 

 umbellatus, Plantago media, and Papaver, cut from the plants,, 

 or with undetached epicotyls of Phaseolns multiflorus, which 

 have developed in darkness to a length of a few centimetres. 

 By means of Indian ink we make fine lines on the objects, 

 at distances of every 10 or 20 mm., to serve as marks, and 

 so divide them into a series of partial zones. The placing of the 

 marks necessitates some care ; it is to be done in the manner 



