308 DR. D. D. CUNNINGHAM ON MYCOIDEA PARASITICA, 
and those which are developed into pollinodia are usually attached to the oogonia towards 
their bases. The contents of the terminal adherent cell appear next to be emptied into 
the oogonium, and to blend with the oosphere. Owing to the reasons previously men- 
tioned, this process has never been actually observed to occur; but the contents of the ` 
pollinodial cells in many eases almost entirely disappear, and many examples of oogonia 
have been met with containing a mass of protoplasm independent of, or only partially 
blended with, the oosphere. 
The oosphere now begins to show a distinct cell-wall, and is soon converted into a 
large spherical or oval oospore, which lies free in the cavity of the oogonium (Pl. XLIII. 
fig. 2). The cell-wall is at first thick and soft, but gradually becomes very thin and 
delicate. Another series of changes has been occurring in the oogonium itself. Its 
position, which, like that of the filaments of the disk, was originally horizontal, has been 
gradually exchanged, in the process of growth, for a more or less vertical one, the base 
of the organ being depressed beneath the plane of the disk, whilst its apex ascends and 
approaches the epidermis of the leaf. Its shape also exhibits alterations, frequently 
becoming more or less acuminate at the apex, and losing its original smooth obovate 
outline. Changes also begin to oceur in the walls. At one point or other towards the 
apex a thinning of the wall begins to occur ; and this process, confined to a limited area, 
gradually advances until the entire thickness is perforated, and a circular opening of 
considerable size and sharply-defined outline (by means of which the interior of the 
oogonium communicates freely with the subepidermal space) is formed (Pl. XLIII. 
fig. 2, & Pl. XLII. fig. 14). 
With this the development of the oogonium itself ceases. In many instances they 
remain in this condition; but in others the mature organs are more or less completely 
invested, save around the ostiolum, by a loose coating of fine filaments, similar in ap- 
pearance and origin to those bearing the pollinodial cells. In any case the original 
filaments of the disk now die off, and the oogonia, with their contained oospores, are left 
persistent among the dried-up débris of the disk beneath the thick epidermis of the leaf. 
How long such oospores may retain their vitality in this condition has not been definitely 
determined; but that they do retain it for a considerable time is certain, as they have 
frequently been obtained in a lively condition from the under surface of dried-up flakes 
of epidermis on the site of old patches of the parasite—where hardly any other evidences 
of its previous presence persisted, and where the leaf-tissues among which they were 
encountered were thoroughly dried up and destroyed (Pl. XLIII. fig. 1). i 
The mature oospores are spherical, and on an average measure about 0:035 to 0-031 : 
millim. in diameter. Sooner or later, when exposed to favourable circumstances, the last _ 
events in the course of their development occur. The dried-up epidermis eventually cracks, 
and in doing so leaves channels of communication open between the external surface of 
the leaf and the interior of the oogonia, as the open mouths of the latter communicate, 
as before mentioned, with the subepidermal space, and in many cases, after the rupture 
of the epidermis has occurred, even project free on the surface (Pl. XLII. fig. 15). On 
the addition of moisture the mass of the oosporic contents now breaks upintoa multitude 
of oval bodies like those developed in the aerial spores; and, the amount of fluid being — 
