INTRODUCTION 



SINCE tlie appearance about thirty years ago of Humphrey's fine 

 monograph on the Saprolegniaceae and the ahnost simulta- 

 neous treatment of the European species by Fischer in Rabenhorst 's 

 Flora, there has been much work done on the water molds, both in this 

 country and Europe. A number of new species have been described, 

 fertilization and other cytological phenomena investigated, and a con- 

 siderable amount of experimenting done on the effect of various media 

 on growth and reproduction. It has been my purpose to get together 

 a treatment of all known species and to describe and illustrate all 

 species I have seen in the living state. Most cytological and physiologi- 

 cal details of importance appearing in the literature since Humphrey's 

 work have been included or referred to under the species involved, and 

 I ha\e not thought it worth while to write a long introduction covering 

 these points. Researches on chondriosomes and our own work on the 

 structure and behavior of the emerging spores will, howe\-er, be re- 

 ferred to in this introduction. 



It will be well to call attention first to several conclusions arrived 

 at by Humphrey that \\a.\e since been shown to be erroneous. In regard 

 to sexual reproduction Humphrey decides from observations by himself 

 and others that fertilization no longer occurs in the Saprolegniaceae. 

 This we now know to be incorrect. A true sexual fusion occurs in Achlya 

 americana \?lv. cambrica, A. polyandra. A. deBaryana, SaproJegnia moiwica, 

 S. mixta, S. didina and Aphanomyces laevis (see these species for details), 

 and it is highly probable that it occurs in a number of others where 

 antheridia are well developed. Credit is due to Trow who in the face 

 of much criticism carried fonvard to a conx'incing conclusion his assertion 

 of sexuality in this family. 



Humphrey was also wrong in his conclusion ('92, p. 77), contrary to 

 the opinion of deBary, Strasburger and Biisgen, that " In general, there 

 can be little doubt that the spores of the genera now under discussion 

 [Saprolegnia, Achlya, Aphanomyces] leave the sporangia automatically." 

 In my opinion the evidence against this view is overwhelming. In the first 

 place, in genera like Achlya and Aphanomyces, where the first swimming 

 stage is suppressed, the spores certainly ha\"e no ciliary action (e\en where 

 cilia are present) sufficient to drive the spores at a high rate of speed through 

 an opening so small as to constrict them. W'here cilia ha\"e been demon- 

 strated within the sporangium their action is very feeble and spasmodic 



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