RESTING-SPORES OF PERONOSPORA INFESTANS, 361 
under great difficulties, ten negatives at least being failures 
before one satisfactory result could be obtained. The diffi- 
culties which presented themselves were manifold, the first. 
and most insuperable being the dull and cloudy weather ; 
this obstacle was made worse by the semi-opaque material in 
which the bodies photographed were found. At sunny times 
characteristic material could seldom or never be found, or 
the photographs would have been better. In looking at an 
object under the microscope the focal distance is constantly 
being slightly altered, so that different depths may be seen, 
but in photography the instruments must be stationary; the 
consequence is that part of the object must always be slightly 
out of focus. Besides this, unless lenses are specially made 
the luminous and chemical rays have a different focus, or 
are not coincident. 
Plate XX, however, sketched by the aid of a camera 
lucida at the time the objects were photographed, furnishes 
a key to the position of the oogonia and antheridia and 
hyphe seen in Plate XIX. The single oogonium at J, lower 
figure, was burst during the moment occupied in taking the 
photograph by the concentrated rays of the sun being sent on 
the object from the reflector of the microscope. ‘To all 
acquainted with similar bodies found amongst other moulds, 
in Algee and some Saprolegniew, these photographs and the 
accompanying key speak for themselves. 
The oogonia, antheridia, hyphe, and mature oospore alike 
in Peronospora infestans are very similar in size, colour, 
shape, and habit to the same organs found in other species 
of Peronospora. ‘he accompanying woodcuts (copied from 
De Bary!) show their appearance in P. alsinearum (Fig. 1) 
and P. umbelliferarum (Fig. 2). The mature resting-spore 
of P. Arenarig is identical in every way with that of P. 
infestans. 
One of the chief points of interest in connection with the 
discoveries made this year rests on the fact of the organisms 
here photographed being in every way identical with the 
bodies found thirty years ago by Dr. Rayer, Chief Physician of 
the H6pital de la Charité, of Paris, and by him communicated 
to Dr. Montagne, who in turn forwarded them to the Rev. 
M. J. Berkeley.? These specimens are still in existence, and 
have been photographed to the same scale as the recently 
1 * Ann. des Se. Nat.,’ 4me sér., xx (1863), pl. 8, figs. 10 and 11; pl. 4,- 
fig. 15. 
2 Montagne described them under the name of Artotrogus hydnosporus, 
They are figured by Berkeley, ‘Journ. Hort. Soc.,’ vol. i, p. 38, t. 4, figs. 
27, 28, 29. 
