Oxygen is one of the constituents of protoplasm and is therefore necessary for all organisms. This does not mean that all organisms must obtain their supply from _free oxygen_, however, as animals and plants generally do. This fact is well illustrated by the differences among bacteria in this respect. Some bacteria _require free oxygen_ for their growth and are therefore called _aërobic_ bacteria or _aërobes_ (sometimes _strict aërobes_, though the adjective is unnecessary). Others _cannot grow in the presence of free oxygen_ and are therefore named _anaërobic bacteria_ or _anaërobes_ (strict is unnecessary). There are still other kinds which may grow either in the presence of free oxygen or in its absence, hence the term _facultative anaërobes_ (usually) is applied to them. The distinction between _facultative aërobe_ and _facultative anaërobe_ might be made. The former means those which grow best in the absence of free oxygen, though capable of growing in its presence, while the latter term means those which grow best in the presence of free oxygen, but are capable of growing in its absence. The amount of oxygen in the atmosphere in which an organism grows may be conveniently expressed in terms of the oxygen pressure, _i.e._, in millimeters of mercury. It is evident that the maximum, minimum and optimum oxygen pressures for anaërobic bacteria are the same, namely, 0 mm. Hg. This is true only for natural conditions, since a number of anaërobic organisms have been gradually accustomed to increasing amounts of O, so that by this process of training they finally grew in ordinary air, that is, at an oxygen pressure of about 150 mm. Hg. (Normal air pressure is 760 mm. Hg. and oxygen makes up one-fifth of the air.) The minimum O pressure for facultative anaërobes is also 0 mm. Hg. Some experiments have been made to determine the limits for aërobes, but on a few organisms only, so that no general conclusions can be drawn from them. To illustrate: _Bacillus subtilis_ (a common “hay bacillus”) will grow at 10 mm. Hg. pressure but not at 5 mm. Hg. It will also grow in compressed oxygen at a pressure of three atmospheres (2280 mm. Hg.), but not at four atmospheres (3040 mm. Hg.), though it is not destroyed. Parodko has determined the oxygen limits for five common organisms as follows: Minimum Maximum. Vol. Mm. In atmospheres. Mm. Hg. per cent. Hg. _Bacterium 1.94 to 2.51 1474 to 1908 0.00016 = 0.0012 fluorescens_ _Sarcina lutea_ 2.51 to 3.18 1908 to 2417 0.00015 = 0.0011 _Proteus vulgaris_ 3.63 to 4.35 2749 to 3306 0 0 _Bacterium coli_ 4.09 to 4.84 3108 to 3478 0 0 _Erythrobacillus 5.45 to 6.32 3152 to 4800 0 0 prodigiosus_