34 On the Resting -Spores of certain Fungi. 



mode of germination takes place only when the spores are en- 

 tirely immersed in water, and in P. curta the exclusion of light 

 is a necessary condition. The germination takes place conse- 

 quently, in most cases, at night, a few hours after the spores 

 are sown. 



In P. umbelliferarum, and the potato mould, P. infestans, if 

 the spores are placed in water, the protoplasm, in the course of 

 a few hours, is divided by delicate lines into distinct portions, in 

 the centre of each of which there is a vacuole (Fig. 2) . The papilla 

 soon vanishes, and after a time the several portions are expelled 

 in the form of perfect zoospores, each of which moves 

 about, when in water, by means of two lash-like threads 

 (Fig. 3). The number of these zoospores varies from six to 

 sixteen in P. infestans, and from six to fourteen in P. umbel- 

 liferarum. The movement of these zoospores ceases in from 

 fifteen to twenty minutes ; each then becomes invested with a 

 membrane, and pushes out a curved, subnexuous, rarely branched 

 thread. Occasionally a second is protruded from the opposite 

 side. In the potato mould the exclusion of light seems more 

 needful than in P. umbelliferarum. 



In P. infestans, the spores, when sown on a%noist substance, 

 put out a simple tube, whose apex swells into an oval, frequently 

 unsymmetrical vesicle, attracting gradually all the protoplasm, 

 and becoming isolated from the mother cell by a partition. 

 This secondary cell may generate a third, and all alike, when 

 plunged in water, are capable of generating the zoospores. A 

 third mode of germination in this species, like that of ordinary 

 spores, occurs occasionally at the apex. The conditions under 

 which the several forms of germination take place, are at pre- 

 sent doubtful. Extended observations can alone show whether 

 peculiar forms of germination or of the development of the 

 acrospores are restricted to certain species. 



In every case these spores are capable of germination or 

 further development the moment they arrive at maturity ; and, 

 if not allowed to become too dry, they may retain their vitality 

 for some days, or even weeks. When germinating upon a 

 plate of glass, their mycelium soon dies ; but if on a leaf be- 

 longing to the particular plant on which the mould is an 

 habitual parasite, or that of some closely allied species or genus, 

 it soon penetrates the leaf, the entering portion becoming at 

 once more or less swollen. The presence of stomates is in 

 general a matter of indifference, though, in some cases, and 

 especially in P. infestans, the germinating threads avail them- 

 selves of the ready aperture of the stomates, notwithstanding 

 their ability to pierce the cuticle. 



In P. umbelliferarum, the germinating thread enters only by 

 the stomates. Fresh spores are formed in about a week or fort- 



