ALKALIES 79 



Cucurbita, &c., can slowly accommodate themselves to very high per- 

 centages of carbon dioxide (over 90 per cent.) l . 



A fact to which no attention has previously been paid is that sudden 

 immersal in pure CO 2 may produce an almost immediate shock-stoppage, 

 from which recovery is possible in air or in CO 2 largely diluted with air, 

 but not in pure carbon dioxide. If, however, the change is a little less 

 rapid, so that the shock-effect is avoided, the stoppage is due partly to the 

 removal of all oxygen, but mainly to the directly poisonous action of the 

 pure carbon dioxide. 



Owing to the influence of the cuticle in retarding diffusion, the stoppage 

 of streaming usually takes two to five, rarely ten or more minutes, in the 

 case of hair-cells of Tradescantia, Cucurbita, Urtica, &c., and a shock-effect 

 is rarely produced, whereas in Vallisneria, Elodea, Trianea, and in pollen- 

 tubes, only from one to two minutes is usually required even in the absence 

 of a shock-effect. A few plants such as Char a and Nitella are, however, 

 inherently more resistant to the poisonous action of the gas in question. 

 Thus streaming may only become slow in cells of these plants after from 

 five to fifteen minutes' immersal, and may not cease until after twenty to 

 thirty minutes. Even then rapid recovery takes place in air, slow streaming 

 being shown in two to five minutes, active in fifteen to thirty. A similar 

 power of recovery is possessed by the plants mentioned above. 



If the exposure is prolonged as far as possible, which in the case of 

 hairs of Tradescantia or Cucurbita may be from two to four hours, there 

 is usually but little increase in the time streaming takes to begin in 

 air (fifteen to ten minutes), but a considerable time, amounting to one or 

 more hours, may elapse before it is fully active, and in some cases it 

 remains permanently slow and ultimately ceases, death ensuing. 



SECTION 34. Alkalies. 



The action of these, on the whole, resembles that of acids. Dutrochet 2 

 was the first to show that sudden immersal in dilute alkalies caused a tem- 

 porary shock-stoppage or retardation of streaming, followed after a longer 

 or shorter period of renewed streaming by a progressive retardation, and 

 ultimately a permanent stoppage. In strong alkalies an immediate per- 

 manent stoppage ensues, according to Dutrochet, although as a matter of 

 fact, by rapid washing recovery may be rendered possible. Moreover, if 

 the concentration is slowly increased, no shock-stoppage but only a pro- 

 gressive retardation is shown. 



Dutrochet :i also found that repeated changes of acidity and alkalinity 



1 topriore, Jahrb. f. wiss. Bot., 1895, Vol. XXVIII, p. 531 ; Samassa, Bot. Zeit., 1898, p. 344. 



2 Ann. d. sci. nat, 1838, ii. sr., T. IX, p. 66. 3 1. c., p. 69. 



