METABOLISM 63 



upon a supply of oxygen. If a transparent cell of 

 plant tissue, in which the protoplasm is in active 

 streaming movement, be so mounted that the sur- 

 rounding air can be replaced by pure hydrogen, the 

 streaming will cease entirely until oxygen is again 

 supplied (the hydrogen is itself inert toward pro- 

 toplasm). Plants likewise cease to grow in the 

 absence of oxygen. The presence of this element, 

 thus appears to be an absolute necessity for life as it 

 exists on the earth to-day. 



But an important exception to this statement must 

 be noted. A number of the lower plant forms have 

 been found to thrive in an absence of oxygen and in- 

 deed to refuse to grow in its presence. Such forms, 

 which include numbers of the bacteria, are called 

 anaerobic. Some bacteria are able to adapt them- 

 selves to either the presence or the absence of oxygen ; 

 others can thrive only in the absence of it. The 

 former are termed facultative anaerobes, the latter 

 obligate anaerobes. Among the latter are numbered 

 some of the most dreaded disease-producing or 

 pathogenic organisms. And in this connection 

 .their anaerobic habit is of much significance. The 

 germ of tetanus (lockjaw), for example, is very 

 widely distributed, and the only reason that the 

 disease is not much more frequent than it is, is that 

 the spores of the active agent cannot develop in the 

 presence of oxygen, and hence only those wounds 

 that are deep and that close over, thus excluding the 

 air, are likely to afford sites for the development 

 of the poison-producing bacteria. 



