Chap. X. ' NATURE OF THE MOTOR IMPULSE. 259 



protoplasm is redissolvecl at the bending -place shortly 

 before the tentacles re-expand, showing that the ex- 

 citing cause of the aggregating process has then quite 

 ceased. Exposure to carbonic acid causes both the 

 latter process and the motor impulse to travel very 

 slowly down the tentacles. We know that the aggre- 

 gating process is delayed in passing through the cell- 

 walls, and we have good reason to believe that this 

 holds good with the motor impulse ; for we can thus 

 understand the different rates of its transmission in a 

 longitudinal and transverse line across the disc. Under 

 a htgh power the first sign of aggregation is the ap- 

 pearance of a cloud, and soon afterwards of extremely 

 fine granules, in the homogeneous purple fluid within 

 the cells ; and this apparently is due to the union of 

 molecules of protoplasm. Now it does not seem an 

 improbable view that the same tendency— namely for 

 the molecules to approach each other— should be com- 

 municated to the inner surfaces of the cell-walls which 

 are in contact with the protoplasm ; and if so, their 

 molecules would approach each other, and the cell-wall 

 would contract. 



To this view it may with truth be objected that 

 wdien leaves are immersed in various strong solu- 

 tions, or are subjected to a heat of above 130° 

 Fahr. (54°4 Cent.), aggregation ensues, but there is 

 no movement. Again, various acids and some other 

 fluids cause rapid movement, but no aggregation, or 

 only of an abnormal nature, or only after a long 

 interval of time ; but as most of these fluids are more 

 or less injurious, they may check or prevent the aggre- 

 gating process by injuring or killing the protoplasm. 

 There is another and more important difference in the 

 two processes : when the glands on the disc are ex- 

 cited, they transmit some influence up the surrounding 



