548 PLANT RESPONSE 



those of the Indian Champaca [Michelia Champaca). The 

 claw of a detached petal is held in a clip, and the petal 

 is immersed in the inverted position in a small glass 

 trough of water, of cubical shape. Inside the trough, 

 and almost touching one face of the petal, say with its 

 own right surface, is a partition of mica, in which is a long 

 narrow slit. Thus the petal is in the left-hand compart- 

 ment of the cube. A microscope of low magnification is 

 focussed on the marginal point of its tip. The required 

 chemical solution is now dropped by means of a pipette 

 into the right-hand compartment. Thus it is diffused 

 through the narrow slit, and acts directly and virtually 

 unilaterally on the proximal side of the petal. If the 

 particular agent should be one which accelerates growth, that 

 is to say induces expansion, a growth-curvature will be 

 induced, the proximal side becoming convex ; l but if its 

 action is to induce depression or retardation of growth, the 

 permanent effect will be a concavity of the proximal. The 

 movements induced are observed by means of the microscope. 



Effect of alkali. — In studying the effect of various drugs 

 on growth in Chapter XXXV. we saw that the characteristic 

 effect of alkali was to produce a contraction and retardation. 

 In the present experiment, on the unilateral application of 

 sodium hydrate in the manner described, the proximal side 

 of the petal was found to become concave, the tip being 

 carried towards the agent. 



Effect of acid. — The general effect of acid was found, as 

 will be remembered, to be opposite to that of alkalis, namely 

 relaxation. On now applying solution of HC1 unilaterally 

 to a petal of Champaka. the result was the convexity of the 

 proximal side. 



Effect of copper sulphate. — This I have found to arrest 

 growth, and its unilateral application in the present case was 

 observed to induce concavity of the proximal side of the petal. 



1 The sudden introduction of a reagent may occasionally give rise to a transient 

 excitatory contraction, but the effects described here are the permanent growth- 

 effects. 



