SAPROLEGNIALES 799 



Matings were made in both liquid and solid media. Diffusion of the hormones 

 across cellophane membranes was shown and the sequence of steps and the 

 timing of each was ingeniously demonstrated by many different types of 

 experiments. Reciprocal matings between A. ambisexualis and A. bisexualis 

 revealed the nonspecificity of certain of the hormones and the specificity of 

 others, the latter providing a precise explanation for the sexual incompatibil- 

 ity of these two species. It was shown that both hormone A and hormone B 

 are stable at 100 C. Intensive studies of the properties and action of hormone 

 A followed [1942a]. It was found that the number of antheridial hyphae 

 produced was proportional to the concentration of hormone A applied and 

 could be used as an accurate index of reaction intensity. Optimal conditions 

 of temperature, hydrogen ion concentration, and other factors were deter- 

 mined, and standard conditions for bioassay of hormone A were described. 

 In 1942, Raper & Haagen-Smit [1942] refined the method of assay still further 

 and succeeded in obtaining, by a complex series of chemical fractionations, a 

 preparation of hormone A 70,000 times more active than the starting material. 

 Unfortunately, despite intensive effort, the substance was not isolated in a pure 

 state, but many of its physical and chemical properties were established. Sub- 

 sequently, Raper [1942b, 1949] discovered that the initial response of the male 

 vegetative plant to the female vegetative plant depends upon a complex of 

 interactions between two substances, A and A 2 , produced by the female, and 

 two substances, A 1 and an inhibitor, produced by the male. 



PHYSIOLOGICAL INVESTIGATIONS 



The classic physiological investigations of Klebs (1896, 1898, 1899, 

 1900) on Saprolegnia, which were followed by those of Kauffman (1908), 

 Obel (1910), Pieters (1915), and others, have furthered enormously our 

 knowledge of the underlying factors determining the growth and repro- 

 duction of members of the Saprolegniales and of aquatic Phycomycetes 

 in general. The conditions necessary to growth have been summarized 

 by Coker from Klebs as follows (see Coker, 1923): 



1 . Uninterrupted continuous growth :— in all good nutrient media, so long 

 as fresh unaltered nutrient is present, e.g., in water with peas, in weak meat 

 extract (1-2%), in gelatin with peptone, in mixtures of water with albumen, 

 casein, etc. 



2. Prompt and complete transformation of the mycelium into sporangia 

 and zoospores : — by placing a well-nourished mycelium in fresh water. 



3. Growth with continuous formation of zoospores :— in very weak solution 

 of certain nutrients, e.g. 0.005% haemoglobin, also in mycelium on agar- 

 albumen jelly that is put in running water. 



4. Active formation of oogonia with limited growth:— by putting a well- 

 nourished mycelium in agar-agar. 



