IRRITABILITY. 567 



such a contraction may take place. Sachs has drawn atten- 

 tion to a case which tends to establish the possibility of a 

 simultaneous increase of the permeability and a contraction 

 of the protoplasm, namely, the cells of Spirogyra. As a pre- 

 liminary to conjugation the primordial utricle of one of these 

 cells contracts away from the cell-wall, and, since it now 

 encloses a much smaller space than it did before contraction, 

 it is clear that, during contraction, a portion of the cell-sap 

 must have escaped through it. 



But whether or not it be admitted that the protoplasm of 

 the cells of a motile organ actually contracts, in the strict 

 sense of the word, in response to a stimulus, we cannot but 

 regard the change, whatever it may be, which the protoplasm 

 undergoes, as a manifestation of that fundamental property 

 of protoplasm which, in the first lecture, we termed con- 

 tractility. It is, like the contraction of muscular fibre, a 

 molecular change accompanied by electrical and thermal 

 phenomena which indicate an evolution of energy on the 

 explosive decomposition of some complex substance ; for we 

 learned in a previous lecture (p. 324) that when the pulvinus 

 of Mimosa or the leaf of Dionaea is stimulated, there is a 

 distinct negative variation of the normal electrical current 

 as there is when a striated muscle is stimulated ; and again 

 (p. 312) that the movement of the petiole of Mimosa is 

 accompanied by a rise of temperature, just as is the contrac- 

 tion of a muscular fibre. 



We will now briefly consider the recovery of the turgid 

 condition of the cell from this point of view. The first step 

 will be the restitution of that condition of the protoplasm "in 

 which it prevents the escape of the cell-sap by filtration under 

 the pressure of the cell-wall, the next, the absorption of water 

 into the vacuole. It is not improbable that, in the chemical 

 decomposition to which we have alluded as taking place in 

 the cell on stimulation, osmotically active substances, such as 

 organic acids, are formed which promote the absorption of 

 water in the process of recovery. Whether or not the pro- 

 toplasm actively expands, or is simply passively distended in 

 consequence of the absorption of water, is a question which 



