382 ELEMENTARY BIOLOGY. [CHAP. 



organism not only result in the building up of its organic 

 substances, but also involve an accumulation of potential 

 energy. If a quantity of dried yeast is burned a definite 

 amount of heat is given off, due to the conversion of this 

 potential energy into the kinetic form. Conversely, the 

 catabolic processes involve a change of potential into kinetic 

 energy. The evolution of heat by actively growing yeast is 

 due to changes connected with its destructive metabolism. 



The Torula being alive, the question arises whether it is 

 an animal or a plant. Although no sharp line of demarca- 

 tion can be drawn between the lowest form of animal and of 

 vegetable life, yet Torula is an indubitable plant, for two 

 reasons. (In the first place, its protoplasm is invested by a 

 cellulose 'coat, and thus has the distinctive character of a 

 vegetable cell. Secondly, it possesses the power of con- 

 structing Protein out of such a compound as Ammonium 

 Tartrate, and this power of manufacturing Protein is dis- 

 tinctively a vegetable peculiarity.^, Torula then is a plant, 

 but it contains neither starch nor 'chlorophyll, and it cannot 

 obtain the whole of its food from inorganic compounds, thus 

 differing widely from the green plants. On the other hand, 

 it is, in these respects, at one with the great group of Fungi. 

 Like many of the latter, its life is wholly independent of 

 light, and in this respect, again, it differs from the green 

 plants. 



Whether Torula is connected with any other form of 

 Fungi is a question which must be left open for the present. 

 It is sufficient to mention the fact that under certain circum- 

 stances some Fungi (e.g. Mucor) may give rise to a kind of 

 Torula different from common yeast. 



The fermentation of the sugar is in some way connected 

 with the living condition of the Torula, and is arrested by all 

 those conditions which destroy its life. At the same time 



