lasne.)]

[Illustration: FIG. 100a.--Formation of conceptacle in _Erysiphe_.]

Certain phenomena concerned in the development of the _Erysiphei_ belong
also to this connection. The mycelium of _Erysiphe cichoracearum_, like
that of other species, consists of branched filaments, crossed in all
directions, which adhere as they climb to the epidermis of the plant on
which the fungus lives as a parasite. The perithecia are engendered
where two filaments cross each other. These swell slightly at this
point, and each emits a process which imitates a nascent branch, and
remains upright on the surface of the epidermis. The process
originating from the inferior filament soon acquires an oval form and
a diameter double that of the filament; then it becomes isolated from
it by a septum, and constitutes a distinct cell, which De Bary[N]
terms an oocyst. The appendage which proceeds from the inferior filament
always adheres intimately to this cell, and elongates into a slender
cylindrical tube, which terminates in an obtuse manner at the summit
of the same cell. At its base it is also limited by a septum, and soon
after another appears a little below its extremity at a point
indicated beforehand by a constriction. This new septum defines a
terminal short obtuse cell, the antheridium, which is thus borne on a
narrow tube like a sort of pedicel. Immediately after the formation of
the antheridia new productions show themselves, both around the oocyst
and within it. Underneath this cell eight or ten tubes are seen to
spring from the filament which bears it; these join themselves by the
sides to each other and to the pedicel of the antheridium, while they
apply their inner face to the oocyst, above which their extremities soon
meet. Each of the tubes is then divided by transverse septa into two
or three distinct cells, and in this manner the cellular walls of the
perithecia come into existence.

During this time the oocyst enlarges and divides, without its being
possible precisely to determine the way in which it happens, into a
central cell and an outer layer, ordinarily simple, of smaller
cells, contiguous to the general enveloping wall. The central cell
becomes the single ascus, which is characteristic of the species,
and the layer which surrounds it constitutes the inner wall of its
perithecium. The only changes afterwards observed are the increase
in size of the perithecium, the production of the root-like filaments
which proceed