8o THE PLANT WORLD. 



cell is expanded — the walls being somewhat elastically extensi- 

 ble — and water, supplied to the general tissue-mass by the veins 

 of the lip, enters. The entrance of water is the result of the 

 expansive tendency, not its cause. As this cell swells, others do 

 so too. The whole upper region of the hinge therefore expands 

 and the hinge gradually straightens, lowering the lip. When the 

 cell and its mates have reached the full measure of this expansi- 

 bihty and become fully turgid the hinge is at its straightest and 

 the lip is in position for the reception of visitors. Now a fly puts 

 foot on the irritable cushion. The irritation travels with great 

 rapidity to the neck cells. Coming to our selected cell the efifect 

 is to release the cell sap, so that some of it flows out into the inter- 

 cellular spaces or into the ducts of the veins. It seems that release 

 must be effected through a sudden change in the confining layer 

 of protoplasm. We may imagine this to relax so that the invisible 

 meshes of its finer structure increase in size and allow the mole- 

 cules of the solubles — hitherto causing distention of the walls — to 

 pass through with ease. Solubles and water — the sap — pass out 

 together, and the cell contracts by its own elasticity. Other cells 

 act simultaneously, and the tissue on this surface of the hinge 

 shrinks, while that on the opposite side remains turgid. The 

 hinge bends and the lip is raised. The great irritability of the 

 cells, including those of the crest and the hinge, is seen when we 

 recall the different kinds of stimuli which aft'ect them : touch, 

 heat, cold, mechanical motion of the parts, and diurnal change of 

 illumination. When the lip is down, the protoplasm of the irri- 

 table parts seems to be in a state of very delicate equilibrium, 

 which may be upset !:>}" trifling changes in the physical conditions.* 



* For Fig. 13, which so well represents the plant, I am indebted to the 

 delineator, Mrs. Oakes Ames. 



