GENERAL MORPHOLOGY AND PHYSIOLOGY 57 



Whereas the period from one septation to another septation is very 

 short, the life cycle referred to is often quite long, perhaps months and, 

 under certain conditions lasting for years. The period of the life cycle 

 can be modified artificially by food supply, chemicals, etc. 



Investigators have succeeded in prolonging the life cycle oiParamecittm.. 

 Normally P. caudatum dies out in about 175 generations; but by applying 

 alcohol (1-5000 to 1-10,000) the cycle has been increased to 860 generations. 

 Very dilute solutions of strychnine gave similar results. If the life cycle or 

 vital impulse of these simple organisms can be prolonged it is probable that 

 similar effects can be produced in higher organisms. Numerous investi- 

 gators have from time to time sought after agents which might inhibit the 

 senile changes in cells and circulatory system (arteriosclerosis) but thus 

 far without conclusive results. 



Microbes feed upon organic substances generally. Those which feed 

 upon dead organic substances are said to be saprophytic; those feeding 

 upon living substances are said to be parasitic. If they can live on dead 

 organic substances only, they are obligatively saprophytic; if they can feed 

 on both dead and living organic substances, they are facultatively sapro- 

 phytic, or, vice versa, facultatively parasitic. The great majority of 

 microbic parasites are facultatively so, as is evidenced by the fact that 

 they can be grown in artificial culture media. Many of the microbic sap- 

 rophytes will develop on living substances under certain conditions, thus 

 showing that they are facultatively parasitic. It is no doubt true that no 

 known microbic parasite actually feeds upon the living substances of the 

 various hosts, since the cytoplasm is in all instances dead before it is taken 

 up and assimilated by the microbe. It would therefore be- more correct to 

 say that parasitic microbes are biologically associated with living organ- 

 isms, while the saprophytes are biologically associated with dead organic 

 substances, and that they all feed upon and assimilate dead organic sub- 

 stances. In certain mutualistic symbioses (as in the root nodules of the 

 Leguminosae) the biological relationship of microbe and host plant is very 

 intimate, but there is no actual interchange of living material. 



All microbes require moisture and warmth (comparatively speaking) 

 for their development, although they are enabled to withstand greater ex- 

 tremes of heat and cold than other organisms. The temperature of liquid 

 air (about 27oF.) does not kill them at once, and the spores may be 

 boiled for some time without destroying their germinating power. Cold 

 (freezing temperature) promptly checks growth and septation, and so does 

 dryness and excessive warmth, although life may not be destroyed. The 

 majority of microbes develop most actively at a temperature of 25C., 

 a few species develop more actively at a lower temperature (2oC.), and a 

 few others at a higher temperature (38C.). Those which develop at a 



