Fungous diseases of plants . following this Tyndall demonstratedthe necessity of discontinuous or successive sterilization, after in-tervals sufficiently long to permit such organisms, or parts of organ-isms, as have remained as spores to germinate, and therefore to bemore readily killed at the next heating. In general, it is necessaryto sterilize on three successive days. As is well known, when oneis careful in making the medium a single sterilization of half anhour is usually sufficient for tubes of agar, if tlie apparatus hasnot become infested with particularly resistant spores. In no case
Fungous diseases of plants . following this Tyndall demonstratedthe necessity of discontinuous or successive sterilization, after in-tervals sufficiently long to permit such organisms, or parts of organ-isms, as have remained as spores to germinate, and therefore to bemore readily killed at the next heating. In general, it is necessaryto sterilize on three successive days. As is well known, when oneis careful in making the medium a single sterilization of half anhour is usually sufficient for tubes of agar, if tlie apparatus hasnot become infested with particularly resistant spores. In no case,however, should one depend upon a single sterilization unless thematerial is to be kept several days previous to inoculation. Stock 18 CULTURE METHODS AND TECHNIQUE quantities of media should be sterilized three times. Between theperiods of sterilization media should be placed at a temperaturefavorable for most bacterial development, and not in a cold place,this being in order that any spores might pass into the vegetative. Fig. sa. Autoclave, Erect Type, Clamped Top, heated by Gas(Photograph by Geo. M. Reed) State within twenty-four hours, and thus be killed at the nextsterilization. A suggestion which has been made by Theobald Smith is ofinterest in connection with the sterilization of media which may con-tain anaerobic bacteria. If such media are sterilized in thin layers,oxygen may have comparatively free access to any submerged ISOLATION AND PURE-CULTURE METHODS 19 spores, and consequently they may not germinate between thesuccessive sterihzations. On the other hand, if the medium isdeep in the vessel, and the exposed surface of the medium small,much less oxygen gains access, and the spores of anaerobic formspass more readily into the vegetative condition and are killed bythe successive sterilizations. Sterilization under pressure. A great time-saving conveniencein sterilization is to be found in the use of the autoclave, or steam ? .- . mi\ m/\ r -^^E^^ _/l fl WM %i^W ?
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