62 SELECTION AND PREPARATION OF ORGANISMS 



tempts to learn the pattern of heredity in animals, he chose the little 

 fruit fly, Drosophila melanogaster, from among more than a million 

 members of the animal kingdom. Drosophila offers some immediately 

 obvious advantages: it is small and easy to handle, it breeds rapidly (one 

 generation about every 14 days), it produces fairly large numbers of 

 offspring from one mating, and w^as soon found to grow readily under 

 the artificial conditions of the laboratory. Later Drosophila was used in 

 experiments which explained the inheritance of sex determination, a 

 result we now know would have been much more difficult to achieve 

 with some other organism. Drosophila also was used to show the relation- 

 ships between chromosomes and heredity, partly because this small fly 

 possesses a special set of "giant chromosomes." It is difficult to give 

 enough emphasis to the contributions of Drosophila to advances in 

 genetics. The collection of Classic Papers in Genetics demonstrates quite 

 clearly the importance of this fortunate choice of organism. 



Another organism enjoying a great reputation in experimental research 

 is Chlorella (mostly Chlorella pyrenoidosa'). Otto Warburg, the German 

 biochemist who was one of the first great contributors in the field of 

 cell physiology and enzyme chemistry, describes in one of his papers his 

 need for an organism for experiments on photosynthesis. He very deliber- 

 ately searched for a plant that would be easy to handle, would grow 

 easily under artificial conditions, would carry on photosynthesis actively, 

 and would be free of a number of annoying complications. After a num- 

 ber of preliminary experiments, he finally settled on this single-celled 

 green alga, Chlorella. Since 1920, Chlorella has been used in a fantastic 

 number of experiments, some quite remote from photosynthesis. Chlo- 

 rella even enjoyed a period of popularity as a possible solution to the 

 world's food problems. Even today, this simple plant is probably used 

 in photosynthesis research, including studies of life support systems for 

 space craft, almost as often as all other kinds of plants combined. 



An example of an unfortunate choice of organisms also comes from 

 research on photosynthesis. One of the first demonstrations that parts 

 of living cells, in this case chloroplasts, can continue at least some of 

 their activity after being separated from the rest of the cell was given by 

 R. Hill about 1938. He ground up spinach, separated the chloroplasts 

 by centrifugation, and then showed that when illuminated under the 

 right conditions these isolated chloroplasts gave off oxygen. Since Hill's 

 original experiment, this ability has been demonstrated in chloroplasts 

 from a variety of plants and under a variety of conditions. Many years 

 earlier, between about 1910 and 1918, Willstatter and Stoll had tried 



