During the year 1861, Dr. de Bary published an account[[8]] of the discovery by him of zoospores, similar to those already described in connection with the conidia of the “white rusts,” produced from the acrospores of the mould which originates the potato disease. In the same author’s memoir of 1864, already quoted, the observations there made are confirmed. When the acrospores of the potato mould and the parsnip mould are sown in water upon a glass slide, their contents become divided, and vacuoles are formed, as already described in Cystopus; these parts are expelled through an apical orifice, and, when free, take the form of perfect zoospores, and commence swimming about in the fluid surrounding them.
[8]. Die gegenwartig herrschende Kartoffelkrankheit, ihre Ursache und ihre Verhütung. Von Dr. A. de Bary. Leipsig: 1861.
These zoospores are oval, or semi-oval, with a structure resembling that of the “white rusts,” save that the two ciliæ, or vibratile hairs, proceed from the same point. The number of zoospores from each acrospore of the potato mould is stated to be from six to sixteen, and in the parsnip mould from six to fourteen. Their ultimate development is the same as has already been described. In the potato mould, the production of the zoospores is much favoured by the exclusion of light.
Another mode of germination in the acrospores of the potato mould has been observed by the same eminent mycologist. This results when the spores are sown upon a humid body, or on the surface of a drop of water. The acrospore emits from its summit a simple tube, the extremity of which swells into an oval vesicle; into this the contents pass, and it isolates itself by a partition from the germ-tube. Thus it becomes a duplicate of the acrospore from whence it was derived. This secondary body has also the power of producing a tertiary cellule in a similar manner. Both the second and third cellule, when immersed in water, produce zoospores in the ordinary manner, as above described.
Yet another and a third mode of germination is described by the same author, in which the acrospore emits from its apex a germ-tube, which elongates considerably; and into this long and tortuous tube the contents of the acrospore pass, and accumulate at the opposite extremity.
The germ-tubes, produced in the manner last described, when developed on the surface of a favourable plant, perforate the cells of the epidermis, or enter by the stomata. In the case of the potato mould, the germ-tubes enter by the stomata; but in the majority of species the germ-tubes do not enter by the natural pores of the leaves upon which the acrospores are sown, but perforate the cellules, and thus effect admission into the tissues of the plant, where they extend, ramify themselves, and become a mycelium. This mycelium originates branched threads, bearing acrospores at the tips of their branchlets, and in many species of Peronospora another kind of reproductive body upon the threads of the mycelium itself. To these bodies we must briefly address ourselves.
This last kind of reproductive organs (not yet positively found in the potato mould) appear to be wholly analogous to the oogonia of the “white rusts” already described, producing oospores in like manner. Dr. de Bary avows with regret that his numerous efforts to observe the germination of these oospores were unsuccessful. Nevertheless, he considers that the perfect resemblance between them and the oospores of the “white rusts” will justify him in concluding that the germination in both is very similar. It will be unnecessary to repeat here the observations already made on the growth and development of oogones and zoospores. What has been advanced respecting these organs in Cystopus will apply also to Peronospora.
Plate XIV.
264.—Potato Mould.
Peronospora infestans.