The Dia-Heliotropic Attitude of Leaves. 



163 



the resulting movement being determined by their combined effects. We 

 may distinguish these four effectors as the upper, the lower, the right and the 

 left effectors. We shall presently find that these different effectors are set in 

 action not merely by direct stimulation, but by the transmitted impulses from 

 a distance, along definite conducting strands by which the different effectors 

 are definitely innervated. 



III. The Nervous Mechanism in Plants. 



I shall now describe the " nervous " mechanism by which stimulus received 

 at the receptive end gives rise to an excitatory impulse, which is conducted 

 along certain definite channels. It is necessary here to justify the use of 

 the terms nervous tissue and nervous impulse in regard to plants, since 

 the idea has long been prevalent that there is nothing in plants which 

 corresponds to the nervous impulse in animals. The transmitted effect of 

 stimulus in Mimosa was thus regarded not as a propagated excitation 

 but merely a hydro-dynamical disturbance. I have shown, however (7), 

 that the transmitted impulse is of an excitatory character, that it may be 

 blocked by various physiological blocks, and that, like the nervous impulse 

 in animals, the velocity of transmission is enhanced with a rise, and 

 •depressed or even arrested by a fall of temperature. 



6. Receptor, Conductor, and Effector. 



The nervous system of plants must be regarded as of a comparatively 

 simple type. In speaking of the evolution of the nervous system, Parker points 

 out tbat the contractile tissue or muscle appeared first as an independent 

 -effector, and that the nerve developed secondarily in conjunction with such 

 muscles as a means of quickly setting them in action ; that a receptor or 

 sense organ alone would be of no service to an organism, neither would 

 nerve or nerve centres alone ; whereas a muscle cell or effector is of use if it 

 •can be stimulated directly (1). 



In plants we find clear indications of these different stages. Thus in the 

 leaf of Erythrina indica, and in the terminal leaflet of Desmodhcm gyrans, 

 the pulvinus is the independent effector, the connecting nerve link being 

 absent or functionally ineffective; heliotropic movement thus takes place 

 when the pulvinus is directly stimulated, illumination of lamina having no 

 effect. In Mimosa and in Helianthus, on the other hand, the intermediate 

 nerve network has, as we shall find, become effective, the leaflets or the 

 lamina serving as receptive organs. Haberlandt (9) has shown that in many 

 •cases the epidermal cells of leaves are of a lenticular shape, for increasing 



