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DISEASES AND AFFECTIONS OF PLANTS. 
* 
The subject, though not perhaps very appropriate to a work peculiarly devoted 
to scientific floriculture, becomes the more interesting at a period when the whole 
kingdom rings with exciting accounts of the potato disease, now unfortunately 
recurrent for a second season, and it is feared under more aggravated circumstances. 
! In a very early number of this Magazine, there was a notice of an erudite paper 
upon a similar subject, read before the Caledonian Horticultural Society. Under 
the head Parasitical Plants on Leaves , there are the following remarks, which are 
extracted verbatim : — “ Leaves are liable to become attacked by various crvptogamic 
plants, from circumstances not well ascertained. The most familiar of these 
diseases is known by the name of mildew {Sporbtrichum macrosporum ) which is at 
once observable by the white appearance it presents on the leaves. Certain circum- 
stances seem particularly favourable to its appearance, as cold, dry weather, and 
particular exposures — plants under the shade of others, or otherwise shaded, 
apparently suffering more than those fully exposed. From microscopical observa- 
tion, this parasitical plant, constituting mildew , seems to be composed of globular 
semi-transparent masses, apparently sometimes attached to a stalk, sometimes to 
hairs on the plants, or collected into heaps on the surface of leaves and stems.” 
The correctness of the above may be disputed so far as concerns the name — - 
since Sporbtrichum is not now considered a mildew : — it is placed in Loudon’s 
catalogue of Fungi, at page 1038 of the Encyclopaedia of Plants, Genus 2478, Species 
16,569, under the division 11 — Mucedines ; whereas the true mildews ( Erycibe ) rank 
under Class 4, Scherotiacei, page 1020. In the list there given, the most familiar 
species are the Erycibe pisi and E. Loniceree, the mildews of the pea and of the 
honeysuckle. These parasites appear as a white mealy suffusion, and under a high 
magnifying power are found to consist of a network of white, semi-transparent 
threads, crossing and intersecting, perhaps by anastomosing processes, in all direc- 
tions. The effect produced upon the pea in autumn, is remarkable ; fructification is 
arrested, and though the plant is not destroyed, yet its vital powers are entirely 
paralysed. The great question of inquiry, and which, as yet, has never been satis- 
factorily met, is this : — -Does the condition of the plant, as to change or disease, 
induce the mildew ; or does the parasite, at its appointed season, fix itself upon its 
victim, and weaken its vital power ? In other words, which is the cause, and 
which the effect ? 
In the potato disease, at that stage of it which is characterised by the dark spot, 
or blotch upon the leaf, a white mealiness is suffused around the spot, extending far 
beyond it, upon the yet green surface, and always on the under side of the leaflet : 
there is no mealiness upon the upper surface, nor any, so far as individual observa- 
tion can be taken in evidence, upon or near the discolorations of the stems. This 
VOL. XIII. NO. CLIII. D D 
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