UNIFORMITY OF EXPERIMENTAL MATERIAL 487 



a frequency of about 50 per day. In C. eUipsoidea and C. pi/renoidosa 

 the waveform has an ampHtude of up to 700% of the minimum and a 

 frequency of 1 per day or less. The large amplitude and low frequency 

 in the Ciilorellas present special problems to the attainment of uni- 

 form experimental material. The amplitude of physiological variation 

 appears less than that of cell volume but heterogeneous material will 

 always result unless cultures are carefully synchronized for division 

 cycle. In conventional batch cultures nonreproducible material may 

 result from unintentional synchronizing and variation in stage of 

 division cycle at harvest unless there is a very strict regimen. 



Two methods of obtaining both homogeneous and reproducible 

 material appear feasible. Tamiya's laboratory has used a technique 

 reminiscent of the training of cells for quantum yield measurements: 

 cultures are de\'eloped at high light intensity and then incubated for 

 7 days at low intensity; resultant cultures have over 90% of small 

 cells of high photosynthetic activity. In our laboratory Sorokin has 

 imposed a careful regimen of light and dark periods upon the other- 

 wise steady-state operation of the continuous-culture apparatus. 

 It appears that the increasing elegance of measurements applied to 

 photosynthesis merits similar attention to uniformity of experimental 

 material. 



Discussion 



Aronoff : I am very glad that, we have a ^iysicin now winch parallels in many re- 

 spects tlu^ advantages of a, young leaf and still possesses the statistical advantages 

 of a large population. In the leaf one has at the moment of its emergence all the 

 cells which the leaf will subsequently have. There is no further mitotic activity. 

 We have conducted age studies on such leaves and have shown that the rate of 

 synthesis of essential amino acids is very different in young leaves as compared 

 to old leaves. It is, therefore, not unreasonable that the composition of such things 

 as proteins is continuously changing throughout the life cycle of leaves, and such a 

 phenomenon, while not unexpected, is in accord with the data that have been 

 found for these algae. 



Tolbert : In older algal cultures, say 3 days old, in a growth flask, if the growth 

 rate is slowed down are there possibly larger cells? 



Myers : In a dense culture two kmds of limitations may develop. A light in- 

 tensity limitation will give rise to a greater proportion of small cells. A nutrient 

 limitation generally has the opposite effect and gives rise to increase in proportion 

 of large cells. 



Tolbert: In much of previous photosynthesis research with Chlorella, algal 

 cultures ranging from 1 to .3 oi- 4 days old have been used after having been grown 



