330 COMPARATIVE ELECTRO-PHYSIOLOGY 



circumstances. The question then suggests itself, what are 

 the circumstances which determine this outflow or inflow ? 

 In the digestive processes, moreover, in plant and animal 

 alike, each of the reactions referred to, whether of secretion 

 or absorption, must be more or less long-continued. Thus 

 these responsive actions, instead of being single and spas- 

 modic, are likely to be multiple and long-sustained. The 

 characteristic response to stimulus of a secreting organ is 

 understood to be by secretion. Is this reaction essentially 

 different from those fundamental processes which underlie 

 the responses of contractile organs, or are we to regard con- 

 traction and secretion as but different expressions of a single 

 responsive phenomenon ? 



In order to test this question, of the connection between 

 responsive secretion and contraction, it will be well here to 

 draw attention to certain experiments of Sachs, on the 

 response of Mimosa, though our inferences will be somewhat 

 different from those which their author intended. If we take 

 a longitudinal slice of the lower half of the pulvinus of 

 Mimosa and keep it in a moist chamber for some time till 

 the tissue has recovered from the excitation due to section, 

 and if we then subject it to fresh excitation, water will be 

 found to ooze out, or undergo secretion from the excited 

 tissue. We may explain this occurrence in either of two 

 ways : first, that, in consequence of the molecular changes 

 induced by stimulus, contraction and permeability-variations 

 take place in the cells, the expulsion of water being an 

 expression of the active process of contraction ; or, secondly, 

 that the oozing-out of the water is a passive process, due to 

 permeability-variation of the turgid cells alone, without con- 

 traction. 



The two theories may be distinguished broadly as those 

 of active contraction and passive secretion. In the thin 

 section of the Mimosa pulvinus it is the exudation of water 

 that is noticeable, and not any marked movement character- 

 istic of contraction. But in the intact pulvinus, owing to its 

 anisotropic structure, the greater contraction of the more 



