174 THE AMEKICAN MONTHLY [August, 



observations on cell-reproduction, since it can be handled with ease, re- 

 sponds quickly, andean be easily observed. Slide cultures on gelatine 

 of various moulds, bacterial organisms, etc., are also easily practised 

 in the laboratory, and yield data on the conditions of cell-reproduction. 

 The follow^ing are among the more important conditions favorable or 

 unfavorable to the grovvith of yeast : a. Food. — Living cells cannot 

 grow unless they are fed. Yeast, if placed in pure water in a warm 

 place, will bud at first vigorously, but this is because the normal cells 

 are never without a surplus of food, and the reproduction soon comes to 

 a stand-still. If yeast be placed in water and sugar and set in a warm 

 place, the process of budding will go on for a longer period, but will 

 soon be retarded and gradually cease, and the yeast cells die. If, how- 

 ever, a nitrogeneous food be added to these, as for example, ammonium 

 tartarate, the budding process is indefinitely kept up. This fact has 

 important bearings and is without exception in the entire range of na- 

 ture. All cells do not require exactly the same kind of food, but all 

 cells must have food, and the green plants, like Protococcus, which can 

 live and grow, and reproduce in distilled water are only an apparent ex- 

 ception, for they anabolize starchy and nitrogeneous foods from gases in 

 the water in which they live. 



a. Tcmperattu-e. — Cells do not reproduce well at all temperatures, 

 but they have a minimum, a maximum, and an optimum temperature 

 at which they reproduce. At points above or below the optimum, 

 their activity is retarded; at points of extreme warmth or cold, they 

 are killed. Some cells, as for instance yeast, can endure great extremes 

 of heat and cold — yeast is not killed by an exposure of lo below zero 

 Fahrenheit for 34 hours, and is not killed by warmth of 312 Fahren- 

 heit applied for only a short time, but is killed by prolonged boiling. 

 The temperature limits vary much for Various cells ; some algae can 

 live in boiling springs. 



b. Light. — Light does not directly promote cell-reproduction. Yeast 

 will grow in the dark as well as in the light, and egg-cells have to 

 grow in darkness for the most part. Para?t2cecium and other infusoria 

 usually live in light places, but there is no proof that the light is 

 necessary to them, and many cells, as for instance those on the inner 

 parts of animals or plants, live and reproduce in darkness. Though 

 light plays such an important part in living organizations, both plant 

 and animal, it is not necessary as are heat and food to all reproduction. 

 We must, of course, observe in this connection that light is necessary 

 to growth in green plants, but these form do not form an exception to 

 the statement that do not require light primarily for purpose of repro- 

 duction. 



c. Moisture. — Water is the natural medium for cell life. Many cells 

 bear indefinite dessication, thanks to certain protections they are fur- 

 nished with, but if they are to reproduce it must be when there is 

 moisture. The power of withstanding drying makes the old style of 

 yeast cakes possible, and also permits the cells of moulds and bacteria 

 to float about as dust in the air, but the germination of these cells, their 

 reproduction by budding or fission, will only go on in a moist place. 

 The cases of plants or animals living in the air are not real exceptions 

 to this law, for the living cells of both these are bathed in fluids, and are 

 killed or suspend animation on exposure to draught. 



