30 Macfarlauc. — Contributions to the History of 



cells of Drosera. His account leads one to believe that he 

 regarded them as closed pits. Sections of Dioncea hair (Plate 

 IV, Fig. 2) show that the free face of each cell is slightly thick- 

 ened, but that where the pit occurs the internal thickening is 

 absent. We cannot as yet say whether each has a pore aper- 

 ture or is a closed membrane, but the knowledge we now have 

 of intercellular connections suggests the former as a likely 



condition. 



One naturally desires to know the use of the minute pits. 

 In proposing an hypothesis we would refer to some of the 

 views that have been advanced to account for leaf closure and 

 subsequent re-expansion. Darwin suggested " that the sev- 

 eral layers of cells that form the lower leaf surface are always 

 in a state of tension, and that it is owing to this mechanical 

 state, aided probably by fresh fluid being attracted into the 

 cells that the lobes begin to separate or expand as soon as the 

 contraction of the upper surface diminishes." Similarly, Bata- 

 lin 1 regards both opening and closing of the leaf as due to a 

 migration of liquids from one zone to another, and considers 

 that the cells of the lower surface are always extremely tur- 

 gid, but that in the expanded state those of the upper surface 

 are even more tensely distended than the former, and he 

 agrees with Darwin that measurements made before and after 

 closure pro^e that contraction of the upper side takes place 

 after stimulation, and he further regards the shutting as a 

 result of disturbance of the tension equilibrium through irrita- 

 tion. In the setting up of this disturbance, he suggests two 

 distinct causes : either (a) an active contraction of the plas- 

 matic substance on the inner, and passive expansion of that 

 on the outer surface, i.e., molecular translocation, or (b), dis- 

 turbance of equilibrium in the tension of the tissues result- 

 ing from expression of water through the walls. He entirely 

 favors the latter or mechanical view, without seeming to 

 think that a combination of the two hypotheses might explain 



matters. 



Recognizing the difficulty of explaining such results as 

 those of Sanderson unless the action of the living protoplasm 



i " Flora," 1877, Nos. 3-10. 



