OF ARTS AND SCIENCES. 73 



Hab. Leaves of Eupatorium piirpureum L., Ambrosia artemisice- 

 folia L., Bldens frondosa L., Rudbcckia laciniata L,, Silphlum tere- 

 binthinaceiim L., Helianthus strumosus L., II. doromcoides Lara., and 

 H. tuberosiis L. 



In the Bussey Bulletin for 1876 less than a dozen Peronosporece 

 were enumerated as known in the United States, but since that date 

 the number has been very much increased. Reference will be made 

 later on to the additions to our flora, but in this connection I would 

 call attention to the large proportion of species whose conidial spores 

 produce zoospores. I have studied the germination in P. vlticola, 

 P. Halstedil, P. obducens, and P. Geranii, no. 218, North American 

 Fungi, and find that the spores all produce zoospores in germinating, 

 although I have experienced difficulty in making those of the last 

 named species germinate, having tried for several j-ears in vain. 

 Besides the species just enumerated, Peronospora (^Basidiophora) 

 entospora Cornu and Roze, which according to Cornu also germi- 

 nates by zoospores, is now known in at least three localities in this 

 country.* Counting also the potato-rot, we have six species which 

 produce zoospores. Of these P. viticola and P. Halstedii are dis- 

 tinctly, and P. entospora probably, of American origin. P. obducens 

 was discovered almost simultaneously in Baden and in Massachusetts, 

 and as it is about equally common on both continents on indigenous 

 species, we have no guide to its original home. P. Geranii, it seems 

 to me, can on morphological grounds with difficulty be distinguished 

 from P. nivea on Umbelliferce in Europe, and I previously regarded it 

 as a variety of that species. Certainly, if there is a difference, I was 

 not able to express it in words. It is, however, distinct from any 

 European form on Geranium, and as the majority of Continental 

 botanists will not admit that the same Peronospora can .grow on 



* The species was originally found on Erigeron Canadense in France, and has 

 since been found in Germany. I first received American specimens from Pro- 

 fessor T. J. Burrill, of Champaign, III., by whom it was found on Erigeron in 

 May, 1878. The species afterwards was described in the Thirty-first Report 

 of the New York State Museum as Peronospora simplex Peck, on Aster Novce- 

 AngHoe, from New York, and it has since been found by Trelease in Wisconsin, 

 on Aster NovcB-Avglice. Although I have often searched, I have never found 

 it near Cambridge, but instead Cercospora caiia Sacc, which to tlie naked eye 

 resembles it. Probably the species is a native of America, and was introduced 

 into Europe with Erigeron, following the e.xample of P. viticola. P. Halstedii, 

 as far as I know, is not yet known in Europe, but it may be e.xpected to appear 

 there at any time, and may do a real injury to the crops of Helianthus tuberosus, 

 which is largely cultivated in several parts of Europe. 



