THE SOURCES OF ENERGY 37 



same time they cause an increased respiratory activity l and an increased 

 production of heat, a portion of the surplus energy apparently finding 

 expression in the induced or accelerated streaming movements. The action 

 is, however, an indirect one, and there is usually no graduated response in 

 answer to stimuli of increasing intensity, the latter suddenly passing from 

 a sub-minimal excitation to a nearly optimal one. 



Streaming seems in all cases to be dependent upon katabolism of some 

 kind or other, but need not necessarily be accompanied by aerobic respira- 

 tion. That the ciliary movements of anaerobic bacteria continue in the 

 absence of oxygen is well known, and in fact the movements of obligate 

 anaerobes may cease in the presence of even small amounts of this gas. It 

 is therefore possible that the cessation of streaming, produced in the cells of 

 aerobic Phanerogams by the absence of oxygen, is merely due to a general 

 effect upon the protoplasmic mechanism, and does not necessarily indicate 

 a direct dependence of streaming upon aerobic metabolism. It is interest- 

 ing in this connexion to notice that different plants exhibit varying powers 

 of maintaining streaming in the temporary absence of oxygen. 



In covered and ringed preparations of leaf-cells of Elodea kept in 

 darkness, streaming ceases in about five minutes, but recommences almost 

 at once when exposed to light, owing to the immediate production of 

 oxygen by photosynthesis. After a prolonged arrest of streaming, how- 

 ever, partial asphyxiation ensues, and the chloroplastids temporarily lose 

 the power of carbon dioxide assimilation, the oxygen necessary for the 

 recommencement of streaming not being evolved until after the lapse of 

 twenty minutes to an hour or more 2 . 



Even in the case of so aerobic a plant as Elodea, a very small amount 

 of oxygen suffices for streaming. Thus I have previously shown (I.e., p. 566) 

 that etiolated chloroplastids, which have at best a very weak power of 

 photosynthesis, may produce sufficient oxygen to maintain slow streaming, 

 and that the latter may continue under a partial pressure of oxygen which 

 does not suffice for the turning green of etiolated chloroplastids. The 

 precise minimal partial pressure of oxygen for streaming has been determined 

 by Clarke to correspond to 1-2 mm. Hg. in the case of Trianea, 2-8 in that of 

 Urtica, and 1-8 in Elodea^. Leaf-sections of Vallisneria are slightly more 

 aerobic than leaves of Elodea, but otherwise behave similarly. Most 

 species of Chara, on the other hand, seem to be partial anaerobes, while in 

 the case of Nitella and Chara foetida the anaerobism is almost complete. 

 This question is of special interest, and hence a more detailed account of it 

 will be given. 



1 Richards, Ann. of Bof.. 1896, Vol. x, p. 531 ; 1897, Vo1 - 

 3 Sea Journ. Linn. Soc., 1896, Vol. xxxr, p. 403. 

 3 Clarke, Ber. d. D. Bot. Ges., 1888, Vol. vi, p. 277. 



XI. 



