104 



THEORETICAL AND GENERAL 



a metronome. In a few cases the rate of propagation was as much as 

 20 mm. per second at 18 C, which in short cells causes a practically 

 simultaneous cessation over the entire cell. 



Hermann (1. c.) observed that an electrical stimulus applied to one 

 end of a cell of Nitella progressively decreased as it travelled along the 

 cell, in some cases becoming sub-minimal before reaching the other end. 

 Apparently, therefore, the stimulus encounters considerable resistance in 

 its passage through the protoplasm, and rapidly loses energy as it travels 

 onwards. Stimuli cannot be increased beyond a certain intensity, for above 

 this intensity the protoplasm is fatally affected. Hence a stimulus of 

 limited duration, however intense, cannot be transmitted beyond a certain 

 distance through undiffentiated protoplasm, and this distance is determined 

 more by the conductivity of the protoplasm than by the character and 

 intensity of the stimulus. 



Transmission from cell to cell. Hormann also concludes that inhibitory 

 stimuli may be transmitted from one cell of Nitella to another by means 

 of interprotoplasmic communications, though none have as yet been seen 

 in this plant. In this respect an analogy would exist with involuntary 

 muscle-tissue, through which stimuli are propagated from fibre to fibre 

 in the absence of motor- nerves. The proof is, however, open to doubt. 

 Electrical, thermal, and mechanical stimuli were used, and in all cases 

 a powerful stimulus is necessary ; the stoppage then occurring frequently 

 almost simultaneously in the two cells. The author has never observed 

 a propagation of locally applied thermal or electrical stimuli beyond the 

 next axial cell, and hence the apparent transmission is probably due to 

 lateral diffusion when an electrical stimulus is used, or when cold is applied, 

 to its direct transmission through the dividing wall. The stimulus is never 

 transmitted to the whorled leaves at the node, probably because of their 

 relatively small points of attachment. Mechanical stimuli may travel 

 through two or three cells, but this is simply because they are rapidly 

 propagated in the form of a pressure wave through the cell-sap, and from 

 cell to cell through the elastic end-walls on which this wave impinges. 



As a matter of fact there appears to be no vital mechanism in plants 

 for the rapid transmission of stimuli from cell to cell, but instead physical 

 means such as hydrostatic disturbances are used. Stimuli may be vitally 

 transmitted from cell to cell by interprotoplasmic communications, but 

 the rate of propagation seems always to be slow, a time-block apparently 

 occurring at each passage from cell to cell. Hence the rate of propagation 

 is more rapid in rows of elongated cells, than in rows of short ones. For 

 example : a stimulus exciting streaming is transmitted more rapidly along 

 the midrib of a leaf of Elodea than through the cells of the lamina. 



A temporary stimulus only produces a very localized effect, since it 

 suffers a steady decrement in passing from cell to cell. If, however, 



