HARDWICKE'S SCIENCE-GOSSIP. 



235 



gas, or some other gaseous impurity which is de- 

 structive to the PeroHospora when it first malces its 

 appearance at the surface of the infected plants, 

 and so stays its further progress. It is well 

 known that lowly-organized plants, such as minute 

 parasitic fungi, are destroyed by applications which, 

 in the case of flowering plants, have little or no 

 effect. Hence, by powdering vines infested with 

 vine-mildew with finely- divided sulphur, which 

 gives off very minute traces of sulphurous acid, -we 

 are able to check the mildew without injuring the 

 vine. But in the case of the potato disease, the 

 difficulty is to isolate the affected plants, so as to 

 prevent their zoospores being dispersed even while 

 they are undergoing treatment. Another difficulty is 

 the extreme rapidity with which the infection takes 

 place : the existence of the disease in any particular 

 case may not be observed till it has reached di- 

 mensions far beyond the application of remedial 

 measures. Under circumstances such as garden- 

 cultivation, however, where there is an opportunity 

 for closer observation, it would be quite worth 

 while, as recommended by Mr. Poynter, trying the 

 effect of the application of soot to the diseased 

 foliage immediately disease is detected. 



It will be seen that in the plants which are 

 attacked secondarily the Perouospora progresses 

 from above downwards, which is precisely the 

 opposite of what occurs in those which are at- 

 tacked primarily. The tubers are consequently not 

 affected till after the haulm. Mr. Parfitt accord- 

 ingly recommended cutting off the diseased haulm, 

 and Mr. Newmareh, writing in the Times of Sep- 

 tember 2nd, states that he found Mr. Parfitt's plan 

 "perfectly successful in preventing the disease. 

 He describes accurately what I did. AVith a pair 

 of hand-shears I cut the tops completely off on the 

 first appearance of the disease, strewing earth over 

 the cut stem, and patting it down with a spade to 

 make it air-tight. In rows so treated I never found 

 a diseased potato ; but though I triumphed in 

 having beaten the disease, I regret to say that the 

 potatoes were utterly worthless except for seed, or 

 for those who do not mind waxy potatoes and 

 attendant indigestion. Still it is something to 

 secure one's seed for next year." Destroying the 

 haulm prevented the^ formation of starch in the 

 foliage, and subsequent storing-up, in the tubers ; 

 the remedy consequently is hardly better than the 

 disease, except in so far as it arrests its progress. 



The Rev. Mr. Moule has started the theory tliat 

 the potato-plant succumbs to the attacks of the 

 Peronospora because it is already in an enfeebled 

 state, from the deficiency of silica in its tissues, 

 owing to the impoverishment with respect to that 

 constituent of the soil on which it is grown. He 

 therefore proposes to use a manure containing 

 water-glass (silicate of soda), which he hopes will 

 give the potato-plant greater vigour, and so enable 



it to outgrow its parasite. Nothing is lost by 

 trying any proposal of this kind, but it is proper to 

 remark that Mr. Moule has not given any proof of 

 the deficiency of silica in the potato haulm ; and 

 there seems no reason to suppose that while cereals 

 can obtain from the same soil a much larger pro- 

 portion of silica than the potato-plant is ever likely 

 to contain, it will suffer from a deficient supply of 

 that element of its mineral food. 



Attention has very reasonably been directed to 

 the inquiry whether it is possible to destroy the 

 resting spores of the Peronospora in the sets of the 

 potato before they are planted. The use of various 

 substances destructive to fungoid organisms has 

 been proposed ; but they all appear open to the ob- 

 jection that if they are made thoroughly to permeate 

 the tissues of the seed potato, they can hardly fail 

 to injure, or at the best enfeeble, the vitality of the 

 buds, or " eyes," as well. Merely to wash the out- 

 side can obviously do nothing towards destroying 

 resting spores, which may be safely out of reach in 

 the interior. 



After reviewing all that has been said on the sub- 

 ject, the only direction in which there seems to be 

 any reasonable hope of relief from the injury and 

 anxiety produced by this constantly recurring 

 scourge, is in obtaining and cultivating early-matur- 

 ing kinds. August is the month in which the potato 

 disease seems to do its worst, especially if the wea- 

 ther be both wet and warm. If the crop can be 

 safely housed before this period, the evil will have 

 been evaded, if it cannot be cured. The production 

 of early kinds is, with the skill and patience which 

 cultivators bring to the systematic improvement of 

 the potato, only a question of time. 



MICROSCOPY. 



Wood Sections.— I think many cuttings with 

 low powers, and merely to show the general forma- 

 tion, are far better seen as opaque objects than as 

 transparent ones. Some look very beautiful in this 

 way, and more natural ; besides, the colour is seen. 

 Nettle root, apple ditto, and clematis canes, are 

 amongst some which are well worth looking at. 

 They must be thin, but not so thin as if used as 

 transparent objects. — B. T. Scott. 



Hot Water Diatoms.— Dr. Blake has collected 

 more than fifty species of diatoms in a hot spring 

 in Pueblo Valley, Nevada, the temperature of 

 which was 163° P. Nearly all were found identical 

 with the species found in beds of infusorial earth 

 in Utah, described by Ehrenberg, showing that the 

 latter must have been accumulated in a hot lake. 

 No other living species were found in the hot 

 water except red algaj. The deposit was a large 

 one, and contained concretions of silica, the 

 greater part of which was made up of petrified 



