G34 STIMULUS-TRANSMITTING SYSTEM 



static pressure within a special system of tubular structures, trans- 

 mission can also be effected in a purely mechanical manner through the 

 equalisation of these pressure-differences. As will be shown later on 

 in detail, stimuli are actually transmitted in this way in Mimosa 

 pudica through the leaf-blades, petioles, and internodes from one 

 pulvinus to another. 



Another way in which a stimulus may be propagated, is by the 

 diffusion of special substances by osmosis from cell to cell from 

 the region first stimulated. If, however, the effects of the so-called 

 " formative " stimuli which lie outside the scope of the present work 

 are left out of account and attention is restricted to motor stimuli, it 

 will be found impossible to cite a single instance in which mechanical, 

 chemical, geotropic or heliotropic stimulation undoubtedly leads to the 

 formation of special substances that propagate the stimulus by their 

 diffusion. 



It is probable that transmission of stimuli most often depends 

 upon the dissemination of some as yet incomprehensible excitation 

 or state of motion, or, to use more general terms, upon the propagation 

 of some physical or chemical change which takes place in the living 

 protoplasts (or in special local extensions thereof). 332 The nervous 

 system of an animal represents a tissue-system specially adapted for 

 this mode of transmission. Among plants, where physiological division 

 of labour has not progressed so far, there seem to be no stimulus- 

 transmitting organs analogous to the nerves of animals. Here, on the 

 other hand, the power of transmitting stimuli is common to every 

 living tissue, provided that its component protoplasts are not com- 

 pletely isolated from one another by cell-walls but remain connected by 

 protoplasmic connecting threads. There can be no doubt that the 

 protoplasmic connections are responsible for the transmission of stimuli 

 from cell to cell ; the comparison so frequently instituted between these 

 structures and the nerves of animals is therefore perfectly legitimate. 



II. PROTOPLASMIC TRANSMISSION OF STIMULI. 



That mode of stimulus-transmission which is directly dependent 

 upon changes within the living protoplasm, may be termed protoplasmic 

 transmission in order to distinguish it both from the purely mechanical 

 and from the osmotic methods. Where the stimulus is not merely trans- 

 mitted across a single protoplast, but is propagated as almost 

 invariably happens among higher plants through a whole series of 

 cells, every path of transmission is made up of sections of two 

 distinct kinds, which alternate regularly with one another, the 

 protoplasts contained in the cell-cavities being responsible for intra- 



