184 THE CAUSES OF SPECIFIC SHAPE 



Since the fibrillae are transitory structures they may favour the transmission 

 of stimuli, and may indeed appear as the result of stimulation, but the best 

 knowledge of the conducting paths will give us no insight into the actual process 

 of conduction. 



We are, however, a step in advance when we are able to ascribe the stimulatory 

 effects to the disturbances produced by the metabolism of neighbouring or remote 

 parts, and by the translocation which the metabolism regulates. In this case we 

 know the means by which the potential differences are produced, and it is in the 

 translocatory channels that these differences are propagated. It is often possible 

 that special substances are used to transmit stimuli, and very probably the changes 

 of shape due to symbiosis are largely produced in this manner, by secretions 

 exuded from the cells for particular purposes. 



In other cases a chemical or physical change may be propagated in a labile 

 system, and instances of such have already been given. Each such reaction is 

 excited only by one or more definite stimuli. Thus the crystallization of a super- 

 saturated solution may be excited either by contact with a crystalline particle of .the 

 same substance or by agitation, a train of gunpowder can be fired by heat, while 

 one of potassium chlorate and sugar can be ignited either by heat or by contact 

 with a drop of sulphuric acid. Hence in a complex living conducting channel, 

 different excitations may excite the propagation of definite stimuli, and so lead 

 to varying results in the end organ. 



Stimuli may be transmitted with or without the aid of living continuity, but 

 naturally, whenever the explosive or conducting substances or systems form an 

 inherent part of the protoplasm, they can only pass from cell to cell through the 

 protoplasmic communications. Transmission within the cell is probably for the 

 most part protoplasmic in character, and the effect of a localized orienting stimulus 

 appears to spread over the cell from the part directly affected. The propagation 

 of a stimulus may involve the transmission of living particles, and may also be 

 accompanied by chemical processes, and by electrical changes due to them. Even 

 in the nerves of animals the electrical phenomena occurring on stimulation are 

 probably the result, and not the cause, of the changes involved in the transmission 

 of the stimulus. 



Irritability and the conduction of stimuli are closely related phenomena. For 

 example a new condition of equilibrium, reached as the result of stimulation, can 

 only be maintained so long as the stimulus remains constant. Hence when an 

 organ returns to its original condition of equilibrium, we have immediate evidence 

 that the internal or external stimuli concerned have ceased to be transmitted to, or 

 to act upon, the part affected. Owing to the excitement of an opposed reaction, and 

 to the resistance offered to propagation, a stimulus is usually transmitted with 

 decreasing intensity and only to a certain distance, even when propagated by some 

 simple chemical or physical change. The activity of readjustment and repair is 

 such as to maintain the conductivity, even when the conduction of a stimulus 

 involves the consumption of an explosive substance, or the liberation of 



1901 ; Hermann, Studien iiber Protoplasmastromung bei den Characeen, 1898, p. 76; Ewart, The 

 Physics and Physiology of Protoplasmic Streaming, 1903, Sect. 47, p. 103. 



