522 RESPIRATION AND FERMENTATION 



the respiratory activity is lessened in buds and tubers during the resting 

 period, and similarly respiration sinks to a minimum in the cells of 

 resting cambium or of dormant primary meristem however richly they may 

 be charged with food-materials. 



A supply of oxygen is essential for the existence of all aerobes, and at 

 the same time the excreted carbon dioxide must be continually removed. 

 Special air-passages and respiratory movements are not always necessary for 

 this purpose, and as a matter of fact do not commonly occur either in the 

 lower plants or in the lower animals. Air may be transferred in the higher 

 plants by means of aeriferous channels to deep-seated tissues (Sects. 29-32), 

 and certain bacteria have even the power of producing pigments which, like 

 haemoglobin, enter into loose combination with oxygen, and thus act as 

 vehicles for the transference of oxygen to the bacterial protoplasm 1 . Physio- 

 logical combustion always produces a certain amount of heat, but plants are 

 devoid of special adaptations for maintaining a constant body temperature, 

 and hence the latter, as in cold-blooded animals, rises and falls with that of 

 the external medium. 



Under favourable conditions respiration may be even more active in 

 plants than in warm-blooded animals : thus in man the carbon dioxide 

 produced in twenty-four hours forms 1-2 per cent, of the body weight, 

 but in mould-fungi it amounts to more than 6 per cent., while bulk for bulk, 

 very active bacteria may consume oxygen 200 times more rapidly than 

 man 2 . Such bacteria and fungi in virtue of their intense disintegratory 

 powers play a most important part in the economy of nature (Sect. 51). 

 The relative respiratory energy is as great in many seedlings as it is in 

 man, provided that they are maintained at the temperature of the human 

 body. As the seedlings grow older, however, more and more cells die or 

 become less active, and hence the respiratory activity of the entire plant 

 or even of single branches is considerably diminished, although the growing 

 embryonic tissue respires as strongly as in the seedling. Similarly old 

 bacterial cultures respire less actively than young ones in which all the 

 cells are rapidly growing and multiplying. Many bacteria, fungi, and 

 lichens exist, however, in which growth is slower and respiration less active, 

 while in certain phanerogams the respiratory activity even under favourable 

 conditions is not more than a fraction of the values given above. 



The greater part, and in adult organs almost the whole, of the nutri- 

 ment is sacrificed in order to obtain a supply of energy, and it is owing 

 to this necessity that seeds germinated in darkness ultimately lose half 



1 Evvart, Journ. of Linn. Soc., 1897, Vol. xxxnr, p. 123 (Evolution of Oxygen from coloured 

 Bacteria). 



1 Vignal, Rev. gen. d. Bot., 1890, T. II, p. 510; Pfeffer, Oxydationsvorgange in lebenden Zellen, 

 1889, p. 475. 



