IRRITO-CONTRACriLlTY IN PLANTS. 205 



aggregation process which Darwin first observed in the cells 

 of Droscra tentacle has been carefully studied, and is now 

 known to be due to expulsion of cell sap through the 

 protoplasmic sac of certain cells, and that this escapes into 

 intercellular spaces in at least some instances, (e.g. Mimosa 

 pudicd), is practically demonstrated by Pfeffer's experiments. 

 The opinion is now being gradually accepted that irritation of 

 the lower region of the pulvinus causes a sudden exudation of 

 the cell sap through special pores in the protoplasmic sac, and 

 that this sap then escapes into the intercellular spaces, or to 

 the exterior if an incision be made. As the cells of the 

 upper pulvinus region are now more turgid, fall of the leaf 

 follows. 1 



But from Wortmann's studies on PJiycomyccs and other 

 plants, it is demonstrated that irritation of any part of a cell 

 causes the protoplasmic sac to retreat from the wall at the 

 irritated region, owing to extrusion of a quantity of the sap 

 that is enclosed within the sac. The process of expansion 

 then would consist in the gradual resorption of sap into the 

 contracted cell. Many results point to this conclusion, and 

 the fact that sensitive plants if starved in their water supply 

 cease to be irritable, is in its favor. 



I previously showed, however, for Dioncea, and have now 

 proved for several other plants, that summation stimuli can 

 be given with definite results ; also, that under heat and cold 

 stimuli, chemical stimuli, and electrical stimuli, plant tissues 

 behave exactly as do the contractile tissues of animals, while 

 the rate of propagation of the stimulus is greater than that in 

 various animal tissues. It may be affirmed, then, of many 

 plants that their protoplasm is irritated by, and responds to, 



1 Since writing the above the author has observed a striking change to occur in 

 the leaflet pulvini of Mimosa ptniica, M. lupulijia Schrankia,angustata, and most 

 beautifully in Mimosa sensitira. After stimulation of a leaflet, and toward the 

 close of the latent period a sudden flush travels centrifugally across the surface 

 of the pulvinus. Immediately thereafter the leaflet contracts, and the pulvinus 

 previously of a whitish hue assumes a dull greenish aspect. The author has 

 grounds for believing that this is due to migration of liquid into the upper region 

 of the pulvinus, and corresponds to a similar change in a girdle of tissue above 

 the swollen leaf pulvinus, which is possibly the area referred to by Lindsay nearly 

 seventy years ago. 



