ZOOLOaY AND BOTANYj MICROSCOPY, ETC. 95 



intensity of respiration varies during development, attaining a 

 maximum during tlie period of germination. 



CO 



For tlie same individual, the fraction * — -— — is the same in light 



and in darkness ; it is also constant under variations of temperature 

 and of presstire. The fraction is in general less than unity while 

 plants are developing rapidly and storing up their food-materials, in 

 the endosperm, cotyledons, rhizomes, bulbs, &c. ; the net result of 

 respiration is then an assimilation of oxygen. But during this period 

 the value of the fraction varies, decreasing, reaching a minimum, and 

 then increasing. 



Respiratory Combustion.t— The experiments of P. Schiitzen- 

 berger were made with a view to ascertain the effect of the presence 

 of certain organic substances on the respiratory combustion of yeast- 

 cells. Similar flasks were filled with equal quantities of water 

 saturated with oxygen, an equal amount of yeast was added 

 to all of them, and then known weights of the particular substances 

 employed, and after the lapse of a given time, the amount of 

 oxygen which had been absorbed was determined by titration. 

 The organic substances added were different varieties of sugar, 

 mannitol, various alcohols, glycerol, acetic, butyric, tartaric and 

 other acids, sodium acetate, Eochelle salt, and other salts, amido- 

 compounds, hydrocyanic acid, and chloroform. Some of these sub- 

 stances have no appreciable effect on the respiratory combustion, 

 others, such as hydrocyanic acid and chloroform, check or retard it 

 considerably. Inverting sugar, ethyl alcohol, and sodium acetate 

 accelerate the absorption of oxygen in a very marked manner, whilst 

 glycerol and the higher homologues of ethyl-alcohol exert a similar 

 but much less energetic action. Methyl-alcohol has little or no 

 influence on the process. The effect of the most active substances is 

 equally well observed with fresh yeast, or with yeast which has been 

 exhausted and washed, but the effect of the less active substances is 

 more clearly observed when the exhausted yeast is employed, because 

 the substances naturally present in the fresh yeast are more combus- 

 tible than those which are added. 



The results show that ethyl-alcohol is particularly apt to undergo 

 slow physiological combustion, its power in this respect being equal 

 to that of inverting sugar. It is possible, indeed, that the inverting 

 sugar is first converted into alcohol and then consumed, and if this be 

 true, ethyl-alcohol and the alkaline acetates must be classed in the 

 first rank amongst those substances which undergo combustion in the 

 living organism. 



Heliotropic and Geotropic Torsion. J — H. Ambronn finds that 

 leaves of Goleus display a torsion of the leaf-stalk as the result of 

 illumination from one side, even when the weight of the lamina of 

 the leaf is counterbalanced. The conditions in which torsions are 



* See infra, p. 104. 



t Comptes Kendus, xcviii. (1884) pp. 1061-4. 



X Ber. Deutsch. Bot. Gesell, ii. (1884) pp. 183-90. 



