CULTURE FOR PHYSIOLOGICAL RESEARCH 47' 



apparatus is built around the use of test tubes which fit into a 

 photoelectric colorimeter so that growth can be followed turbidi- 

 metrically directly in the culture vessel. The apparatus is illus- 

 trated in Figure 2. 



Test tubes 21.5 mm. by 175 mm. are used as culture vessels. 

 Agitation, and carbon dioxide supply are provided by passing 

 in an air : carbon dioxide mixture (95: 5) through a bubbling 

 tube. The bubbling tube is constricted about 50 mm. below 

 its upper end in order to hold a small cotton plug which is 

 inserted into the test tube through a larger cotton plug protected 

 by a gauze wrapping. The 5% carbon dioxide gas mixture is pre- 

 pared by allowing compressed air and compressed carbon dioxide 

 under constant pressure to pass through orifices so chosen as to 

 obtain the correct mixture. Flow rates of about 50 cc. per sec. 

 for air and 2.5 cc. per sec. for carbon dioxide are indicated by 

 Venturi flow gauges. The gas mixture is held at about 2 lb. per 

 sq. in. pressure by allowing some of the excess gas to escape 

 against a head of water. Manifolds providing 24 outlets per bath 

 have been constructed from brass tubing and inexpensive needle 

 valves (Imperial No. 103). The water baths are built from V2 in. 

 by 6 in. aluminum stock, channelled to allow fitting of plate 

 glass sides; they are 22 in. long by 7V2 in. deep and accommodate 

 24 tubes each. Thermostated 25° C. water is pumped through 

 the baths continuously. Illumination at a level of about 600 f.c. is^ 

 provided by two banks of four 60-Vv'att tungsten lamps, one bank 

 on each side. Subsequent work has shown that two 20-Watt 

 fluorescent lamps on each side also provide adequate illuminationi 

 with less heating. 



The test tubes easily accommodate 10 ml. of culture medium^- 

 which may be sterilized by autoclaving. In short-time growth 

 experiments, however, it is often possible to dispense with sterile- 

 precautions. Growth is estimated as optical density in the Evelyn 

 photoelectric colorimeter using a 600 mu. filter, the bubbling 

 tube being raised about an inch during the reading. For any 

 given suspension of Chlorella pyrenoidosa Link the optical den- 

 sity is proportional to population up to a density of about i.o 

 which corresponds approximately to 2.0 cmm. cells per ml. op. — ....^ 



