HARDWICKE'S SCIENCE-GOSSIP. 



267 



atmosphere to become very damp, then the absorb- 

 ing power of the mycelium is very much increased, 

 ■while the assimilating power of the leaf-cells is 

 little altered. Thus it happens that a sudden change 

 from dry weather to moist will cause the mycelium 

 to increase so very much beyond the power of the 

 potato-plant to support it, that in the struggle for 

 existence the latter blackens and dies." " When 

 the disease has arrived at a certain point, viz. just 

 about the time of the appearance of the spots on 

 the leaves, these mycelial threads make their way 

 into the air" (p. 323). 



I give in conclusion an illustration of the perfectly 

 mature resting spore of Peronospora infestans, as 

 seen imbedded in the substance of the potato-leaf. 

 These resting spores, which carry on the winter life 

 of the fungus, are not restricted to the leaves, for I 

 find them sparingly in both haulm and tuber, al- 

 though I have at present seen the best specimens in 

 the leaves. The engraving given herewith (fig. 164) 

 shows a transverse section through a black spot of 

 one of the leaves from Chiswick, and the resting 

 spore is seen at a, nestling in amongst the cells of 

 the leaf. An antheridium, b, and two oogonia 

 (c, c), from which such resting spores arise, may 

 be seen in the cut, and the old common form of the 

 fungus will be noticed breaking through a hair on 

 the upper surface of the leaf, which is a very un- 

 common occurrence. The situation of the resting 

 spores can generally be ascertained on the leaves 

 by noticing the slightly thickened and very dark 

 spots, for the bodies are commonly in these spots. 

 It is, however, an extremely difficult matter either 

 to get them out, or, indeed, to see them when im- 

 bedded, for, when mature, they are black-brown in 

 colour, and only a little larger in size than the leaf- 

 cells. These leaf-cells are also intense brown-black 

 in colour, from contact with the hurtful mycelium, 

 and almost as hard as wood. The best way to see 

 the resting spores is to macerate the leaves for 

 several days in water, and then set them free by 

 crushing tbe spot between two slips of glass. The 

 presence of the fungus in the leaf makes the cells 

 very thick and woody as well as black, so that in 

 crushing the leaf-cells the resting spore is not un- 

 commonly crushed at the same time. With care, 

 however, they can be got at, when they will be 

 seen, as at d, covered with warts or coarse reticu- 

 lations, and beautifully regular and perfect in out. 

 line : whenfyoung they are of a pure warm sienna 

 colour, and when perfectly mature, brown-black 

 and shining. They are spherical or slightly egg- 

 shaped, and measure on an average about one- 

 thousandth of an inch in diameter. I consider it 

 worthy of special note that these resting spores are 

 almost exactly the same in size, conformation, and 

 colour with Peronospora arenarice, Berk., an allied 

 species found parasitic on Arenaria trinervis. In 

 looking for these bodies care must be taken not to 



confound them with corroded cells, granules of 

 starch injured by the disease, or foreign bodies. 



At e is shown a semi-mature resting spore with 

 pollinodium attached, accidentally half washed out 

 of its coating of cellulose by maceration in water. 



I may say, as an addendum, that to me there is a 

 marked analogy in size and habit on the one hand 

 between the oogonia and the vesicles which contain 

 the zoospores, and on the other hand between the 

 simple spores and the antheridia. I consider that 

 the oogonia and antheridia are merely the inter- 

 cellular condition of the vesicles which contain the 

 zoospores and conidia, which latter are the aerial 

 state of the former. 



The facts which point in the direction just indi- 

 cated are these : sometimes there is no differentia- 

 tion in the contents of the vesicles, but the plasma 

 is discharged in one mass and not in the zoospore 

 condition; the vesicle then resembles the oogonium. 

 At other times the oogonium shows a distinct dif- 

 ferentiation in its contents, and matures from one 

 to three resting spores, which to me shows an 

 approach to the condition of the vesicle which 

 usually gives birth to the zoospores. — See also the 

 Gardener's Chronicle, July 17 and 24, from which 

 the above plates have been taken. 



Since the above observations were printed, the 

 following facts have been observed by me, and re- 

 corded in the Gardener's Chronicle for July 31. 



1. Some plants sent to the Royal Horticultural 

 Society by Mr. Dean, on July 21, were covered with 

 the Peronospora far beyond anything I had ever 

 seen before. The haulm, the leaves (on both sides 

 alike), and the berries were covered. Some of these 

 plants, after being placed on a garden bed, and 

 covered with leaves (to keep them moist), were the 

 next day one white mass with the Peronospora. 



2. The potato fungus (as commonly seen) bears a 

 far larger number of simple spores than inflated 

 vesicles containing the zoospores or swarm-spores, 

 but in Mr. Dean's plants the fungus produced 

 zoospores almost exclusively, and in the greatest 

 abundance. As the zoospore is a higher develop- 

 ment of the plant than the simple spore, this latter 

 observation points to the unusually robust health ol 

 the fungus this season. 



3. Ou suspending the infected leaves over a glass 

 of water for from twelve to seventy-two hours, the 

 swarm-spores fell in abundance (either free or in 

 the vesicle) on to the water, and there germinated. 

 No single drop of the water could be taken up for 

 examination without meeting with the germinating 

 spores, the threads radiating over the water in every 

 direction, evidently in quite a congenial element. 

 It brought the following fact to light, which is of 

 importance : some of the vesicles which usually 

 discharge the zoospores discharged instead a thick 

 mass of mycelium ; and this cord, when it had pro- 

 ceeded a considerable distance over the water, there 



n 2 



