Witson: NortH AMERICAN PERONOSPORALES 65 
about 40-55 4; epispore thick, verrucose, or with low blunt ridges 
which are often confluent and irregularly branched. 
This is the most widely distributed and by far the commonest 
species of the genus. Occurring as it does upon such a large 
number of hosts, a wide variation in characters is to be expected, 
yet an examination of numerous specimens, both American and .- 
foreign, has shown a remarkable stability of essential characters. 
The fungus as it grows upon Bursa, from which it was originally 
described, does not differ materially, either in habit or measure- 
ments, from that upon other species of Brassicaceae. In Europe 
the same fungus attacks various species of Capparidaceae and 
exhibits the same characters. An A/bugo which occurs in Europe 
upon eseda has also been referred to this species, from which it 
differs materially in habit, producing a much thinner and more 
superficial sorus than those produced upon the other two families 
of hosts. In the absence of odspores and of perceptible difference 
in the conidia this disposition of the material had best be retained. 
The point of greatest variation in the species in America is that of 
oospore-formation. The odspores have not been observed on the 
majority of hosts and their location varies greatly in cases where 
they are known. Odspores have been examined from the follow- 
ing hosts: Brassica nigra (stems), Bursa Bursa-pastoris (capsules), 
Camelina microcarpa (leaves), Raphanus sativus (capsules), Roripa 
Armoracia (leaves) and Sophia pinnata (leaves), Swingle* also 
reports odspores from Dentaria diphylla (leaves), Bursa Bursa- 
pastoris (stems) and Lepidium campestre (stems). The morphology 
of this species has been studied by Wager + and Stevens. } 
On BRASSICACEAE: 
Arabis furcata S. Wats., Montana, Rydberg & Bessey 4230; 
Washington, Suhsdorf 266. 
Arabis lyrata L., New York, Underwood. 
Arabis virginica (L. ) Trel., Alabama, Underwood ; Mississippi, 
Tracy. ae ; 
Barbarea Barbarea (L.) MacM., California, Heller 570 
(Fungi Columb. ia 
it 
*Jour. Myc. 7; 110, 111. 
tAnn. Bot. 10: 297-342. tes is aoe 1896. 
T Bot. Gaz. 32: 91, 98, 254. pl. 2. 1901. 
