2Q2 LECTURE XIV. 



growing. The consideration of the mechanics of growth may 

 be conveniently deferred for the present : that subject will be 

 treated of in a subsequent lecture. 



It is easy to prove that growth involves an expenditure 

 of energy, inasmuch as it will only go on when destructive 

 metabolism is active, that is, when the evolution of energy is 

 considerable. It is a well-known fact that the higher plants 

 do not grow in the absence of free oxygen. Malpighi found 

 that seeds would not germinate in water which was covered 

 with a layer of oil, under conditions, that is, in which they 

 could obtain no oxygen : Senebier and de Saussure ascer- 

 tained that branches will not grow in vacua, nor in an atmo- 

 sphere which does not contain oxygen : more recently Detmer 

 has shewn that the growth of seedlings is arrested when they 

 are deprived of oxygen, and his results have been confirmed 

 by Wortmann. We know that oxygen is essential to the 

 normal destructive metabolism of these plants ; the obvious 

 inference is that growth does not go on in the absence of 

 oxygen because the destructive metabolism is too feeble, that 

 is, that the evolution of energy is inadequate. In the case of 

 plants which can live without oxygen, growth will proceed 

 provided that the destructive metabolism is sufficiently ener- 

 getic. With regard to Yeast (Saccharomyces cerevisicz) it has 

 been much debated whether or not the cells can grow and 

 multiply in the absence of oxygen. Considering the activity 

 of the fermentation which it can excite, it would seem pro- 

 bable a priori that it can. This assumption is on the whole 

 confirmed by the experimental evidence. It appears, from 

 the researches of Pasteur, of Brefeld, and of Nageli, that, as a 

 matter of fact, the Yeast-cells do grow in the absence of 

 oxygen, provided that they are supplied with fermentable 

 material (sugar), and that the alcohol, the principal waste- 

 product of the metabolism of the cells, is not allowed to accu- 

 mulate. Amongst the Schizomycetes many forms are, as we 

 have seen (p. 211), anaerobiotic. Clostridium butyricum, for 

 example, the plant which causes butyric fermentation, not 

 only grows and multiplies in the absence of oxygen, but, as 

 Pasteur and Prazmowski have ascertained, it dies when free 



