NEGATIVE HELIOTROPISM 



599 



more excitable. In this last case, we may obtain a very 

 pronounced negative response in consequence of the relatively 

 greater natural excitability of the distal side. 



We shall see in the course of this and the following 

 chapters how heliotropic movements other than positive are 

 actually brought about under these different conditions, and 

 in the present chapter we shall study cases which are illustrative 

 of the first two. 



Fig. 244. Microscope Recorder 



Plant mounted in cubical glass trough with root in water. Light strikes 

 root, R, unilaterally from the right side. Movement of root observed 

 by microscope, M, the inclined transparent disc of glass, G, giving at 

 the same time the reflected image of the recording pen, P. 



Recording microscope. — Since the growing root has to 

 be kept in water, for the purpose of studying the phases of its 

 responsive curvature, the method hitherto employed of obtain- 

 ing records by the Optic Lever is inapplicable. I therefore 

 devised a different method of observation — that of the Record- 

 ing Microscope (fig. 244). The method of record will be 

 understood from the figure, where in a cubical glass trough 

 a piece of the stem of Bindweed, with its water root, R, is 

 securely fixed on the surface of the water. Light is made to 



