DEPENDENCE OF THE CURVATURE ON GROWTH. 685 



Within this growing region then, as has already been mentioned, the rapidity of 

 growth is so distributed that it increases from the bud up to a certain distance from 

 it, reaches a maximum, and then declines further back from the bud, and finally passes 

 over into the region of fully grown parts. In this connection stems with many joints 

 which do not form pronounced nodes (e. g. Asparagus) behave like individual long 

 internodes, such as the scapes of the Leek. If, on the contrary, the shoot-axis is 

 sharply segmented into separate internodes, as in the case of Polygonum Sieboldi, 

 each internode exhibits its own curve of partial growths which increase upwards from 

 the node next below, attain a maximum at some one place, and again decrease up to the 

 node next above. The form of the geotropic curve in this case suffers interruptions at 

 the nodes. Apart from these matters, however, all that has been said of single long 

 internodes or that is to be said of shoot-axes with many leaves and no joints, applies 

 in general and in detail to a jointed stem. 



To understand the form of the geotropic curvature, however, a few other points 

 have still to be mentioned. In the first place, every transverse zone of a grow- 

 ing stem first begins to grow slowly, then grows more rapidly and reaches a 

 maximum, and then slackens in growth till it finally ceases. The more rapid the 

 growth is at one place, the more pronounced the curvature it experiences through 

 geotropism. The rapidity, however, with which the geotropic curvature occurs, 

 depends in addition essentially upon the thickness and elasticity of the part capable of 

 curvature, since it is obvious that a thick shoot in order to make the same curvature 

 as a thinner one, must suffer a greater difference in the length of the convex and 

 concave sides, for which a relatively longer time is necessary, and that this also needs 

 a greater expenditure of force where the elasticity is greater. Moreover the curva- 

 ture which actually takes place at any place on the shoot-axis depends upon how 

 long this has been submitted to the influence of gravity transverse to its longitudinal 

 axis, and, further, the after-effect which makes itself especially marked in the case of 

 geotropic curvature comes into consideration. Shoots or single internodes which have 

 been lying horizontally for one or two hours without curving perceptibly, may, when 

 subsequendy placed erect or fixed on a klinostat, become curved eventually in con- 

 sequence of the geotropic influence to which they were previously subjected. 



Finally, in criticising the form of curvature exhibited after some time by an ortho- 

 tropic shoot laid horizontally, in consequence of geotropism, the circumstance has 

 to be considered at what angle the vertical cuts the longitudinal axis of the shoot. 

 Experiment teaches that a shoot which is at first vertical and then placed obliquely, 

 is less strongly affected geotropically than if it had been laid horizontally : the 

 geotropic stimulation is the greater the nearer the inclination of the shoot-axis to the 

 vertical approaches a right-angle. If a shoot with a long growing region is laid 

 horizontally, and geotropic curvature then begins, the anterior portions situated 

 nearer to the bud will soon become erect, not only on account of their own geo- 

 tropism, but also passively on account of the curving of the older portions, and by 

 this means they come into a position in which gravitation is acting only at an acute 

 angle, and thus exerts a more feeble geotropic action. 



The co-operation of these very different factors brings it about that the form of 

 the curvature, so long as the geotropic movement is taking place at all, is continually 

 changing, even in the same shoot, and that it is different in different shoots. Only the 



