J26 LIFE : OUTLINES OF GENERAL BIOLOGY 



plants is now recognised by all, and for driving this conclusion home 

 by persistent and ingenious experiments great credit is due to Sir 

 Jagadis Chunder Bosc. See, for instance, his Nervous Mechanism in 

 Plants (1926). 



When the glowing but not flaming tip of the long thin incense 

 stick of the Chinese is aj)plied to a secondary rib of the beautiful 

 compound pinnate leaf of the Sensitive Plant, Mimosa pudica, the 

 result is a spreading impulse, reminding one of the nervous impulse 

 in an animal. The leaflets close together from below upwards; the 

 secondary ribs or stalks draw together as in a folding fan; the leaf 

 as a whole suiks down when the stimulus reaches the motor cushion 



Fig. 46. 



Leaf of the Sensitive Plant {Mimosa pudica). i, fully expanded; 2, in pendent 

 position, with pinnules folded upwards. 



or pulvinus at the base of the main leaf-stalk; the impulse passes 

 into the stem and other leaves collapse; if the stimulus be strong 

 enough, the impulse, whatever it is, may ascend to the apex of the 

 stem and travel down the other side. Of course, there is nothing but 

 convenience in using an incense stick; a scratching pin will do as 

 well provided that the stimulus is superficial. It must not injure 

 the surface, nor cause exudation of sap, nor irritate the enclosed 

 wood. The central problem is the conduction of the excitation from 

 the place stimulated to a distance in the plant where a motor organ 

 or pulvinus is activated. Is the conduction like that which passes 

 along the nerve-fibres of animals ? 



Bose comes to the deliberate conclusion that the higher plants 

 have a well-defined nervous system. Excitation from part to part is 

 conducted by the phloem or bast of the fibro-vascular bundle. The 

 excitation is not due to the movement of fluid in vessels or cells, as 

 in the sap-currents; it is of the nature of a protoplasmic thrill, as in 



