444 PHYSIOLOGY OF GROWTH. 



may be accurately specified. The oldest pieces of the haulm 

 no longer curve at all. The form assumed by grass haulms which 

 have raised themselves geotropically, is shown in Fig. 144. We 

 can easily satisfy ourselves by experiment that not only intact 

 haulms, but also haulms longitudinally split are capable of curving 

 geotropically. 



If some sections of grass haulms, each with a node at its centre 

 (I experimented with Hordeum), are stuck horizontally into moist 

 sand, and others of the same age obliquely upwards, we find after 

 one to two days that the former have curved more vigorously than 

 the latter, as is at once shown by measuring the angles. From 

 this experiment, which may be repeated with any stems which re- 

 act energetically to the stimulus of gravity, it follows that the 

 effect of gravitation on plants is greater, the more nearly the angle 

 at which it acts approaches a right angle. 



FIG. 144. Piece of a grass haulm, curved geotropically. 



Finally we will make a number of experiments on the growth of 

 geotropically curving grass nodes and other plant structures. It 

 is well known that under normal conditions the nodes, after attain- 

 ing a certain stage of development, cease to grow. Strangely, how- 

 ever, growth re-commences in the cells of the node, when the 

 grass haulms are placed in a horizontal position. 



We cut out lengths of rye or barley haulms, each with a node at 

 its middle, mark the length of the nodes on two sides by means of 

 fine ink-lines, and place the objects in our zinc box. After two or 

 three days we again measure the distance between the ink lines on 

 the geotropically curved pieces of haulm, using a strip of paper 

 divided into millimetres. It is seen that the convex lower side of 

 the node has increased considerably in length, while the concave 

 upper side has become shorter owing to the compression of its 



