234 BULLETIN OF THE BUSSEY INSTITUTION. 
ton specimens contained odspores, but we have not found them in 
European specimens. As far as we can ascertain, P. alta differs from 
P. effusa principally in its taller and less frequently branching conidial- 
stalks. 
The most interesting Peronospora recently recorded in the United 
States is P. obducens. Schroeter. It was first observed by Mr. B. D. 
Halsted on the cotyledons of Impatiens fulva, Nutt., growing in the 
Bussey brook, May, 1876, and found the following week, by the writer, . 
at Newton, Mass. Since then, the species has been observed several 
times by the writer on Jmpatiens fulva, the under surface of whose 
cotyledons and young leaves it covers with a thick white down. The 
fungus disappears early in June; the seedlings which are severely 
attacked being killed at once, and others recovering from the attack, 
so that the fungus is no longer to be seen, at least on the surface, after 
the cotyledons have fallen. ‘The mycelium of the fungus is distinctly 
seen throughout the cotyledoris, and, owing to the frequent inter- 
cellular spaces, presents a more striking appearance than that of 
almost any other species of Peronospora with which we are acquainted. 
The hyphe are broad, and branch in all directions. Haustoria are 
numerous, and of a spheroidal shape, attached by a narrow neck as 
are the haustoria of Cystopus candidus, from which, however, they 
differ in being very much larger. ‘The ordinary diameter of the 
haustoria is .01 mm., but, as they grow old, they become, not unfre- 
quently, .02 mm. in diameter and the cell wall, which is at first thin 
and colorless, becomes thick, yellow, and roughened, so that, when rather 
thick sections of the leaf are examined, the first impression is that one 
has before him a number of small-sized odspores. 
The mycelium, which is most beautifully seen in the cotyledons and 
first pair of leaves, is also found in the young stem, but is by no means 
so striking there as in the less compact tissue of the cotyledons. The 
conidia are borne on stalks which are about .8 mm. high, and which 
penetrate, few in number, through the stomata; the dense felt produced 
by the fungus on the lower surface of the cotyledons being caused by 
the great number of stomata, through which the conidial stalks make 
their way, rather than by the number of stalks which pass through each 
stoma. ‘The conidial stalk rises nearly straight, and gives off three or 
four principal branches at right angles. The latter again branch ina 
similar manner three or four times, the ultimate divisions being some- 
