150 CARL DOWNEY LA RUE 



tion of the three central cells of the spore which appears only at maturity, 

 and serves as a criterion for the elimination of mere growth stages; (7) the 

 total absence of any sexual form of reproduction. 



EXPERIMENTS 



Methods and conditions 



Since the aim of the investigation was to study variations of genetical 

 significance, every reasonable attempt was made either to remove varia- 

 tions in environment or to provide controls for them. All the cultures 

 used were grown on agar in tubes, and under ordinary laboratory condi- 

 tions. Aside from the fact that it would be extremely inconvenient, if not 

 impossible, to control a selection experiment in which the organism was 

 grown as a parasite on its usual host, such a procedure would doubtless 

 offer more variations as to food supply, etc., than would growth on culture 

 media. WOLLENWEBER (1914) has shown that organisms which are 

 facultative parasites show as normal development in culture as on their 

 usual hosts. 



That fungi, especially Saprolegnias, may be exceedingly sensitive to 

 variation in food supply has been shown by KAUFFMAN (1908) and PIET- 

 ERS (1915). However it is not believed that Pestalozzia is so exquisitely 

 sensitive to such variations as are some other fungi. In any event the 

 variations thus introduced into this work are doubtless much smaller than 

 those which of necessity apply to any other organism which has been used 

 in the study of selection up to the present time. 



The nutrient solution was all made up before the experiment was begun. 

 Tender twig tips and young leaves of Hevea brasiliensis were boiled in 

 water and a quantity of brown sugar made from the sugar palm (Arenga 

 saccharifera) was added. The decoction was strained, concentrated by 

 boiling, and after thorough mixing to secure uniformity, was poured into 

 flasks which were plugged with cotton stoppers and sterilized. 



The single lot of medium thus prepared was stored and used throughout 

 the experiments until the writer's return to the United States. (A supply 

 of the medium was brought to America but was lost in transit, so that it 

 was necessary to supply a substitute, as will be noted later.) For growing 

 the cultures nutrient agar was made up according to the following formula: 

 Hevea decoction, 250 cc 

 Water, 3750 cc 



Agar-agar, 120 gm 



