[Illustration: FIG. 28.--_Bacillus megaterium_: _a_ outline of a living, vegetative cell-rod; _b_ a living, motile, pair of rods; _p_ a similar 4-celled rod after the effects of iodine alcohol; _c_ a 5-celled rod in the first stages of spore-formation; _d-f_ successive stages of spore-formation in one and the same pair of rods (in the course of an afternoon); _r_ a rod with mature spores; _g^1–g^3_ three stages of a 5-celled rod, with spores sown in nutritive solution; _h^1–h^2_, _i_, _k_, _l_ stages of germination; _m_ a rod in the act of transverse division, grown out from a spore which had been sown eight hours previously. (After de Bary; _a_ mag. 250, the other figures 600 times).] [Illustration: FIG. 29.--_Bacillus amylobacter._ Motile rods, partly cylindrical and without spores, partly swollen into various special shapes and with spore-formation in the swelling. _s_ Mature spore, with thick mucilaginous envelope. (After de Bary; mag. 600 times, with the exception of _s_, which is more highly magnified.)] The functions of life cease on a slight excess of the maximum or minimum temperature, numbness setting in when either of these limits is passed. _Crenothrix_-threads provided with mucilaginous envelopes may, according to Zopf, sustain a temperature of-10°. Some Bacteria are said to be able to resist the exposure to as low a temperature as-110° for a short time. It is not known at what degree of cold the death of the Bacteria occurs: the greatest degree of heat which the vegetative cells can withstand is about the same as that for other vegetative plant-cells, namely, about 50–60° C. Certain Bacteria, _e.g. B. thermophilus_, grow and thrive vigorously at 70° C. Many spores, on the contrary, are able to bear far higher temperatures (in several species a temperature for some duration of above 100°, those of _Bacillus subtilis_, for instance, can withstand for hours a temperature of 100° in nutrient solutions; the spores remain capable of development after exposure to a dry heat of 123° C.). The _Desiccation_ of the air, if prolonged, kills many forms when in the vegetative condition. The spores however can bear a much longer period of dryness, some even several years.