107 
Peronospora hydropiiylli. 
(Plate XVII, Figs. 17-24.) 
Spots yellowish on both sides of the leaf, but more visible from above, 
2 to 4 imn broad by 10 to 25 mm long, with rather indistinct margins, lim¬ 
ited laterally by the veins, or by confluence covering the greater part 
of the leaf, becoming brown with age or causing the whole leaf to shrivel 
up. Under surface of the spots sparsely frosted by the conidiophores. 
Mycelial liyplue occasionally branching, quite irregular, narrowed 
down at frequent intervals to half the average diameter and covered 
with protuberances, some of which are sufficiently extended to form 
branches; diameter varying from 6 to 7/4 in the constricted portions to 
15 or 16/4, or even more, in the swollen parts, averaging 10 to 12//. 
Haustoria small, 15 to 24// long, consisting of a short, broadly clav- 
ate branch with three to five finger-like small branches arranged in a 
whorl around its apex. 
Conidiophores rather large, dichotomously many times branched, the 
branches bent into reversed curves and often twisted around each 
other; tips slender, tapering to a blunt point, curved or often with a 
reversed curve, numbering sixteen in a small specimen to fifty on an 
average, or to seventy-five on a very large one. Total length of the 
conidiophore varying from 200 to 450//, length of the stem to the first 
branch varying from one-half to four-sevenths the total length. The 
first branch is relatively large, usually from two-thirds to three-fourths 
of the length of the whole head, and contains about that proportion of 
the tips, often nearly equaling the rest of the head. 
Conidia ovate, without apical papillae or marks to indicate the point 
of attachment; smoky colored, measuring 19 by 28// to 25 by 35//, aver¬ 
aging about 21 by 30//; germinating by means of a lateral tube which 
is often curved in a loose spiral and usually at one or two points swollen 
abruptly to twice the normal size, which is gradually resumed again. 
Oospores xiroduced in the leaf x>arenchyma, subglobose, 39 to 45// in 
diameter, light brownish in color; endospore rather thick; episxiore 
thin in places, causing the margin of the sj>ore to become undulate in 
section. Walls of the oogonium thin. 
On Hydropliyllum Virginicum , L. Oregon, Ill., June 1, 1888. Herb. 
M. B. Waite, No. 558. Near Washington, D. C., May 5, 1889, No. 559, a 
single infected xdant. Iowa City, Iowa, spring, 1888. A. S. Hitchcock.* 
Observations .—It will be seen from the description and drawings that 
this Sfiecies is a typical Peronospora of the section Effuscv. It firesents 
no difficulties in classification unless it be in the fact that the sxiecies 
in the section Pffusce are not very clearly defined, and botanists are 
obliged to defend mainly on the host plants for the determination of 
the species. The germination of the conidia was accomxdished by 
* These specimens are mentioned, hut not described; by McBride and Hitchcock, in 
Bull. No. 1, from the Laboratories of Nat. Hist, of the State Univ. of Iowa, p. 51. 
