RESPIRATION AND OXYGEN. 91 



Summary. — The absence of oxygen stops the movement of proto- 

 plasm in all plant-cells studied, with the possible exception of Nitella. 

 This is equally true whether the air be removed by exhaustion, re- 

 placed by hydrogen, or its access prevented in various ways. Since 

 Ewart has shown that exceedingly minute quantities of oxygen suffice 

 for streaming in Chara, it is probable that this is likewise the case 

 with Nitella and that the movement in the latter is not really anae- 

 robic. Carbon dioxid and nitrous oxid inhibited protoplasmic move- 

 ment in every instance, but both Lopriore and Samassa found a 

 gradual accommodation to the former, so that higher and higher 

 percentages were required to check streaming. As to the effect of 

 an increased oxygen-content, Demoor thought that movement was 

 promoted by it, but Lopriore found this to be true only of slow 

 streaming, and Samassa obtained no increase at all. 



With respect to the movements involved in mitosis, Demoor 

 observed that mitotic processes continued in the stamen-hairs of 

 Tradescantia after carbon dioxid had caused streaming to cease. No 

 cell-plate was formed, however, and division remained incomplete 

 as long as movement was impossible. He concluded that the activ- 

 ity of the nucleus is independent of that of the protoplasm, but it 

 seems that this can be but partially true at most, since mitosis is 

 imperfect in the absence of oxygen, and streaming comprises but a 

 part of the protoplasmic activity. 



Sabline obtained similar results with root-tips of Vicia grown 

 without oxygen, the cell-plate being absent and the spindle faint, 

 while Samassa found that the withdrawal of oxygen inhibited mitosis 

 as well as movement. This discrepancy may be explained by the 

 results of O. Nabokich, who found that different plants and different 

 parts of the same plant varied much in sensibility to the lack of 

 oxygen. Mitotic figures disappeared in the sunflower only with 50 

 hours' exposure, but earlier in the buds of pea and bean, while in 

 root-tips of the pea they were absent after 20 hours and in those of 

 the bean after 5 hours. In most cases the cell-plate appeared in the 

 sunflower, but it was suppressed after 5 hours exposure in the bean. 

 The conclusion that nuclear division is independent of oxygen and that 

 the injuries observed were due to secondary factors does not seem to 

 be warranted by the facts. It seems not improbable that mitosis 

 is still possible in the presence of minute quantities of oxygen, as 

 Ewart has shown for streaming in Chara, and that such amounts 

 may have persisted in the tissues for the relatively short periods of 

 exposure. This explanation is supported by Pfeffer's observation 

 that movement continued for some time after the exclusion of oxygen 



IRRITABILITY. 



The first observations upon the relation of oxygen to irritability 

 were those of Dutrochet (1838), who found that the leaflets of 

 Mimosa pudica folded together with the beginning of air exhaustion, 



