78 PHYSIOLOGY OF STREAMING MOVEMENTS 



Elodea is less sensitive, but after twenty-four hours' immersal in 

 o-oi per cent. H 3 PO 4 (or a shorter period in stronger solutions) streaming 

 entirely ceases, but recommences in water in from five minutes to an 

 hour, then slowly regaining its original activity. 



Most organic acids act similarly, though not with the same vigour. 

 Thus a i per cent, solution of oxalic acid corresponds approximately to 

 a 0-05 per cent, solution of HNO 3 , HC1, or HgSO^. 1 Streaming, for 

 example, persists in Trianea for one and a half hours after the application 

 of oxalic acid in concentrations, increasing up to I per cent. 



SECTION 33. Carbon Dioxide and Carbonic Acid. 



The presence of carbon dioxide, either as a gas or in solution, will 

 necessarily diminish the amount of oxygen available under ordinary cir- 

 cumstances. Hence the poisonous character of carbon dioxide can only be 

 correctly determined by experiments in air containing varying percentages 

 of carbon dioxide and similar percentages of an indifferent gas, combined 

 with experiments in which the carbon dioxide replaces a portion of the 

 nitrogen of the air, the percentage of oxygen remaining constant. It is of 

 interest to notice that the resistant power of different organisms varies 

 considerably, and that as we descend the scale in both plant and animal 

 kingdoms, we meet with organisms having great powers of resistance 

 (Mosses, Bacteria, lower Worms, certain Infusoria 2 ), together with others 

 which are comparatively sensitive (obligately aerobic Bacteria, Infusoria, 

 and Fungi). 



The presence of a cuticle, even if thin, retards the poisonous action of 

 carbon dioxide. Thus Kiihne found that streaming ceased in hair-cells of 

 Tradescantia only after forty-five minutes in pure carbon dioxide, and 

 became active again after half an hour in air, whereas Klemm observed an 

 almost immediate cessation of streaming when hairs of Trianea or leaf-cells 

 of Vallisneria were suddenly immersed in CO 2 , or in water saturated with 

 this gas 3 . According to Lopriore, however, streaming does not cease until 

 after two to three minutes' immersal in carbon dioxide in the case of hairs 

 of Cucurbita and Tradescantia, and after it has been allowed to recommence 

 in air it cannot be stopped again even by a current of pure CO 2 lasting for 

 five hours. This was undoubtedly due to the fact that the gas used by 

 Lopriore contained an appreciable quantity of oxygen, while, as Lopriore 

 and Samassa have shown, the cuticularized hair-cells of Tradescantia, 



1 Klemm, 1. c., p. 36. 



2 C. Frankel, Zeitschr. f. Hygiene, 1889, Bd. v, p. 332 ; Arsonval, Compt. rend., 18*91, T. CXil, 

 p. 667. 



3 Kiihne, Unters. iiber das Protoplasma und die Contractilitat, Leipzig, 1864, p. 106; Klemm, 

 1. c., p. 36. 



