SELECTION WITHIN PURE LINES OF PESTALOZZIA 151 



The medium was very thoroughly mixed before being put into tubes, and 

 the same amount was placed in each tube. In making cultures care was 

 taken that all the tubes of a given generation were poured from agar of 

 the same lot. 



The cultures were not kept in a constant-temperature chamber but the 

 range of variation in temperature and humidity is not great in the east 

 coast region of Sumatra. For the greater part of the year the daily change 

 in temperature is from about 23 C to about 32 C. The total range of 

 fluctuation for one five-month period of the selection experiment was from 

 19.4 to 33.9 C. During the same period the moisture content of the air 

 varied from 52 percent to 100 percent with a usual daily range of approxi- 

 mately from 60 percent to 100 percent. In general the climate of Asahan 

 varies so little from day to day, or from month to month, that it would 

 be hard to find any place more uniform. This uniformity allows cultures 

 to be carried on indefinitely without danger of extremes of heat or cold 

 such as have ended various experiments of other investigators. 



The selection experiments were made according to two plans. In the 

 major part of the work the line of descent was determined wholly by 

 divergence in progeny means without regard to the visible characters of 

 the spores chosen as generation parents. In other words, the direction of 

 selection in the chief experiments was guided by the performance of particu- 

 lar spores, rather than by their appearance, but the method was checked 

 by parallel experiments in which the usual basis of selection, namely, 

 selection of visibly divergent individual spores, was employed. 



The characters concerned were length of spore and length of spore 

 appendage. That these characters are heritable to a very considerable 

 degree is shown by the fact that strains, distinct for either spore length or 

 for length of spore appendages, have been isolated, and that these strains 

 remain distinct from generation to generation in spite of fluctuations in 

 environmental conditions. The heritable differences between the different 

 strains are very considerable as may be seen from figures 1 and 2, and these 

 differences make them especially promising for use in a study of the results 

 of selection. 



The number of cells in the spore and the number of appendages per 

 spore might appear to be characters suited for study but this was soon 

 found not to be the case. The number of cells in the spore shows little 

 variation; the smallest number found in observing hundreds of thousands 

 of spores was three, and the largest number seen was nine. In two cases 

 where spores having one more than the normal number of cells were 



GENETICS 7: Mr 1922 



