TWENTY years since, and some of these little pests were altogether unknown, whilst others were only recognized and partly understood by a few scientific men. During the period to which we have alluded more than half the present species contained in the genus Peronospora had never been observed, and amongst these the most devastating of its tribe, the associate and undoubted cause of the potato disease.

Parasitic fungi are far more numerous, both in individuals and species, than most persons are aware, and cultivated plants of all kinds are more or less subject to their ravages. Some are more susceptible than others, of which the corn and grass tribe, or GraminaceÆ, as they are termed by botanists, is an example. Not less than thirty species have been recorded upon plants of this natural order, and of these nearly one-half are found upon the living plants. Upon the potato plant, again, no less than ten different kinds of fungi have been described; whilst upon other and more fortunate plants only one or two parasites of this nature establish themselves.

It will be sufficient for our present purpose to state that one of the six families into which fungi are divided for scientific purposes is called Hyphomycetes, a name compounded of two Greek words signifying “thread” and “mould,” or “fungus,” and is applied to this group because the thread-like filaments of which they are largely composed are the most prominent feature. In this family there are again a number of smaller groups called orders, having an equal value to the natural orders of flowering plants; and one of these orders, called Mucedines, has the fertile threads perfectly distinct from the mycelium or spawn. These threads are sometimes simple and sometimes branched; they may be articulated or without articulations or septa, short or long, erect or creeping, hyaline or whitish, mostly free from colour, and are not coated with a distinct membrane. The spores are generally simple, sometimes solitary, at others in pairs, or strung together like beads for a necklace. Amongst all this variety of arrangement there is order, for these are but features, or partly the features, of the different genera of which the Mucedines are composed. One of the genera is termed Peronospora, and to this the parasitic fungus of the potato, and some others to which we shall have occasion to refer, belong. In this genus the threads are generally branched, but without articulations. The spores, or seed-like bodies, are of two kinds; one kind is borne on the tips of the branches; and the other kind, which is larger and globose, is borne upon the creeping mycelium or spawn. All the members of this genus with which we are acquainted are parasitic on living plants, inducing in them speedy decay, but preceding that decay of which they are themselves the cause. Hence we have deemed it the more advantageous course both for writer and reader to associate together the different species of this particular genus of parasitic moulds in one chapter, rather than bring together the different kinds of fungi, belonging perhaps to widely separated genera, but all associated with, or parasitic upon, the same plant. The botanical student will thank us for following this plan, and the general reader will labour under no disadvantage, in this instance at least, from the similarity of the diseases produced in the plants infested.

It has been recently proposed to associate the genera Peronospora and Cystopus together in one group, under the name of Peronosporei; but with the discussion of this subject we have nothing to do in this volume. Having announced this fact, we shall continue to notice them in the several positions heretofore accorded to them.

The mycelium (root-like fibres) in this genus greatly resembles that described for the “white rusts,” though neither so thick, nor is the membrane so gelatinous as in that genus. In some instances the mycelium is confined to the inter-cellulary passages; but in most cases they also perforate the cells of the plant which nourishes them. The sucker-like bodies already described in Cystopus are often found produced on the mycelium of Peronospora, but occasionally they appear to be absent, especially in the mould causing the potato disease.

From the mycelium erect threads are produced, upon which one form of fruit, which may be termed “acrospores,” is borne. These filaments are sometimes single, and sometimes in small tufts or fascicles. In some instances they are considerably branched, so as to present a dendroidal or tree-like appearance; in others they are nearly simple, being only surmounted by short spicules; or, in one instance, quite simple, and only surmounted by a single acrospore. The branching habit is by far the most common.

Each ultimate branch in the ramification of the fertile filaments engenders a single acrospore. Its extremity, at first thin and pointed, swells in the form of a globular vesicle, which soon takes the elliptical or ovate shape of the perfected acrospore, and at length separates itself from the branch that supports it.

In all instances the acrospores have a similar structure, but with minute differences in form, &c., which have their importance in the determination of species. In most cases the apex of the acrospore is obtuse, and the entire body has a violet tint, more or less deep; in some it is completely colourless. These acrospores, when placed in favourable conditions, will germinate, and, in fact, comport themselves in the manner of true spores.

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.

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.

Potato Mould.—Towards the close of the summer of 1845, in the course of a few weeks, every one became aware of the fact that a new disease had appeared which threatened the entire destruction of the potato crop. Until then it seemed to have been almost, although not entirely unknown. It first appeared in the Isle of Wight about the middle of August, and a week afterwards had become general in the South of England, and the next week there were but few sound samples of potatoes in the London market. Early in September the disease had commenced its ravages in Ireland, and shortly afterwards it was discovered in Scotland. With the same rapidity it seems to have spread throughout Europe and North America, or at least the western portion of the former and the northern districts of the latter. It must not be imagined, however, that the Isle of Wight was the centre from which this disease spread over such an extended area and with such alarming rapidity. From this spot it doubtless made its first appearance that year amongst our own crops, but there is not the least doubt of its existence both on the continent of Europe and in North America in the previous year, and the farmers of Belgium had noted its appearance in the province of LiÈge as far back as 1842 and 1843. Other diseases had been observed affecting the potato crop before, and one which was also associated with a parasitic fungus had made its appearance in 1815. It is also exceedingly probable that, in a milder form, the murrain was present with us a year or two before it broke out to such an alarming extent. A correspondent to the Gardeners’ Chronicle, in 1844, notices it in the Isle of Thanet, and another testifies to its occurrence in districts of Ireland for two or three years previous to its general outbreak. The description of the disease in Canada, in 1844, contained in a letter addressed to Dr. Bellingham, and quoted by the Rev. M. J. Berkeley,^[9] leaves no doubt of its identity:—“During the months of July and August (1844), we had repeated and heavy showers, with oppressive heat, and an atmosphere strongly charged with electricity. Towards the close of the month of August I observed the leaves to be marked with black spots, as if ink had been sprinkled over them. They began to wither, emitting a peculiar, offensive odour; and before a fortnight the field, which had been singularly luxuriant, and almost rank, became arid and dried up, as if by a severe frost. I had the potatoes dug out during the month of September, when about two-thirds were either positively rotten, partially decayed and swarming with worms, or spotted with brownish-coloured patches, resembling flesh that had been frost-bitten. These parts were soft to the touch, and upon the decayed potatoes I observed a whitish substance like mould.”

Although this disease made its first appearance, in the middle of August, 1845, in the Isle of Wight, it had already appeared in Belgium in the same year, a month previously; and although it may have been noticed in other British localities in 1844, it was known in Canada and in St. Helena in the same year to a far greater extent, and in LiÈge as early as 1842. There are, therefore, good grounds for believing that the European centre was Belgium; but if M. Boussingault was correct in stating that “this malady is well known in rainy years at Bogota, where the Indians live almost entirely on potatoes,” then it is not of European but American origin, and is probably derived from districts not far remote from those whence Europe first received the potato itself.

It would occupy too much space to detail the different theories and opinions relative to the causes of this disease to which 1845 and subsequent years gave birth. Suffice it to say, that the lapse of years has silently proved the majority of these to have been fallacious. All such as imputed to peculiar electric conditions, a wet season, or other meteorological influences, the disease which has re-appeared under different conditions and influences, and in seasons remarkable for dryness, are manifestly refuted; whilst its mycological origin has continued to gain adherents, and the gradual accumulation of fresh facts has almost placed it beyond dispute not only that the potato disease is accompanied by, but results from, fungal growth. Unfortunately, this disease has been so prevalent, more or less, during the past eighteen years, that few have been without the opportunity of making themselves acquainted with its external appearance. To this may be added the minute and exact account of its development, as recorded by that excellent mycologist and careful observer, the Rev. M. J. Berkeley, in 1846, and to which, even now, nothing of importance can be supplemented or abstracted:—“The leaves began suddenly to assume a paler, and at length a yellowish tint, exhibiting here and there discoloured spots. More or less coinciding with these spots, on the reverse of the leaves, appeared white mealy patches, consisting of a minute mould, proceeding, either singly or in fascicles, from the stomata, and arising from an abundant branched mycelium creeping in every direction through the loose tissue beneath the cuticle. The upper surface rarely, if ever, exhibits the mould, it being almost physically impossible for its delicate threads to penetrate the closely-packed cells which, being arranged side by side, leave scarcely any intercellular passages. The mould, in a few hours from its first piercing the apertures of the stomata, perfects its fruit, and in so doing completely exhausts the matrix, which in consequence withers. No sooner have a number of the leaves been attacked, than the stem itself is subject to change, becoming spotted here and there with dark brown patches, in which the cells are mostly filled with a dark grumous mass, without exhibiting any mucedinous filaments; though, occasionally, I have ascertained their presence. Very rarely fructifying but dwarfed specimens of the mould occur upon it. The stem now rapidly putrefies, the cuticle and its subjacent tissue become pulpy, and separate when touched from the woody parts beneath. The whole soon dries up, and in many instances exhibits in the centre the black, irregular fungoid masses which are known under the name of Sclerotium varium, and which are believed to be the mycelium of certain moulds in a high state of condensation.

“If the tubers are now examined, the greater part will often be found smaller than usual, especially if the disease has commenced at an early stage of growth; but in their natural condition, while here and there a tuber, particularly if it has been partially exposed, exhibits traces of disease. The surface is, however, soon marked with livid patches, commencing generally about the eyes, or at the point of connection with the fructifying shoots: these rapidly acquire a spotted appearance, the spots being rather waved, and assuming often a more or less concentric arrangement. Sometimes—especially on the smoother kinds of tuber—two or more regular systems of concentric spots are exhibited on the same tuber. The skin now withers, and is easily separated; the spots become depressed and of a yellowish tinge; and if the tubers be laid in a moist place, in a day or two—sometimes in a few hours—the same mould which destroyed the leaves springs from them, piercing the cuticle from within, yet not scattered, as on the leaves, but forming a conspicuous white tuft. If a section of the diseased tuber be made on the first symptoms of the disease, little brownish or rusty specks are found in the cellular tissue, confined, with very rare exceptions, to the space between the cuticle and the sac, if I may so call it, of spiral vessels and their accompanying tissue, which, springing from the subterranean branches, pass into the tuber, making their way to the several buds disposed on the surface. These spots consist at first of a quantity of discoloured cells, mixed more or less with others in a healthy condition. The grains of fecula are for a long time perfectly healthy; the cells themselves, so far from being looser, are more closely bound together than in the more healthy portions. The rusty spots soon exhibit a darker tint, spreading in every direction and becoming confluent; they at length extend beyond the barrier of vascular tissue, and attack the central mass. The tuber, meanwhile, assumes a disagreeable smell, decomposes more or less rapidly, other Fungi establish themselves on the surface, or in the decaying mass, which emits a highly fetid odour, resembling that of decaying agarics; the union of the cells is dissolved, animalcules or mites make their appearance, till at last the whole becomes a loathsome mass of putrescence.”

The form of the mould itself is represented (fig. 264) as exhibited under the microscope, with the nodose swellings of the branches, and spores attached to the tips. These acrospores are filled with a granular mass, from which, as hereafter described, zoospores are produced. The branching dendroidal threads of this fungus proceed from a creeping mycelium or spawn of entangled filaments which interpenetrates the matrix, upon which it establishes itself. Upon these threads spherical bodies were long since observed by Dr. Payen, and, under the name of Artotrogus, described by Dr. Montagne as a new species of fungus. Dr. de Bary failed to detect oospores on the mycelium of this mould, although the organs found by Payen in some sort resemble them. De Bary sought in vain, also, for the spherical bodies described by the author above named. It has been calculated that one square line of the under surface of the leaves is capable of producing 3,270 acrospores, each of which yields at least six zoospores, sometimes double that number; thus we have 19,620 reproductive bodies from that small space. The mycelium from the zoospores is capable of penetrating the cellular tissue in twelve hours, and, when established there, it bursts through the stomata of the leaves, and fruit is perfected in from fifteen to eighteen hours. Since the zoospores are perfected and ready to germinate in twenty-four hours from their being placed in water, it becomes almost impossible to calculate the myriads of fungi that may be produced from a single centre. Dr. de Bary has also demonstrated that the brown spots so characteristic of the disease are the result of the action of the spores or zoospores. By placing a quantity of spores in a drop of water on the leaves, stems, and tubers under a glass sufficiently air-tight to prevent evaporation, he produced the brown spots, and traced their progress from the earliest stages.

There are a few practical conclusions which may be drawn from these discoveries. In the first place, it is clearly shown by the production of the spots that the fungus is capable of causing the disease, a fact which has been disputed, but now placed beyond doubt. The inference is, that not only is it capable of producing, but is really the cause of the potato murrain. With bodies so minute and active as the zoospores, there can no longer be difficulty in accounting for their penetrating the tissues of the plant. They are most active and productive in wet weather, especially when it is also warm. Moisture appears to be essential, and a dry season the greatest enemy to the spread of the disease. That bodies so minute and subtle should have baffled all efforts to destroy or eradicate, is not now surprising. Whether any method will be found to contend successfully with it, is now more doubtful than ever. A careful reperusal of the old facts by the aid of this new light will tend to the elucidation of much of the mystery in which the subject has been involved. All who have hitherto been sceptical of the mycological source of one of the greatest pests of modern times should study M. de Bary’s pamphlet.

The potato mould has been judiciously named Peronospora infestans, or, as it was at first called, Botrytis infestans; but on a revision of the genera Botrytis and Peronospora, it was transferred to the latter genus, in which it remains. Three names were given to it, within a short period of each other, by different mycologists, in ignorance of its having already received a name. The one we have adopted appears to have the priority, at least of publication, and was given by Dr. Montagne. That of Botrytis devastatrix was given by Madame Libert, and Botrytis fallax by M. DesmaziÈres. The principal feature in this species seems to consist in the branches becoming alternately thickened and constricted, so as to resemble a moniliform string or necklace of little bladders or vesicles. The branches are also more erect than in the allied species, and the spores are solitary on the tips or from the sides of the branches, and not in pairs or clusters, and the tips are simple, and not bifid or trifid, as in most of its allies. It need scarcely be remarked, that a high power of the microscope is necessary to make out the distinctive features of the different members of this genus, and that to the naked eye they only appear as a minute whitish mould. As already stated, this little fungus makes its first appearance on the under surface of the leaves, especially the lower ones, of the potato plant, and afterwards attacks the stem, and ultimately the tuber. For examination it is better to select the leaves soon after the fungus makes its appearance.

Turnip Mould.—Since the advent of the potato murrain a similar disease has been witnessed, though more limited in its extent, amongst Swedish turnips, commencing in little waved irregular lines following the course of the vessels, around which spots are formed by the deposition of dark granules in the same manner as in the potato. In this instance, the leaves apparently are first attacked in a similar manner by a species of mould or Peronospora allied to the one already described, but which has been long known as parasitic upon cruciferous plants, to which the turnip belongs. This species, termed Peronospora parasitica, is white in all stages of its growth. It is much more branched, and the branches are comparatively shorter than in the potato mould, and the tips of the branches are bifid (fig. 262). The acrospores are very large and globose, features also which distinguish this mould from the last. A short time since we were called to witness a bed of splendid cauliflowers, which had, up to that time, been the pride of their cultivator; but, alas! their glory was threatened with speedy annihilation, for in nearly every instance the lower leaves had become more or less covered on their upper surface with yellow spots, and beneath glaucous with the mould we have been describing. The diseased leaves were all immediately removed, but we fear without success, although no positive information has since reached us. The almost unnatural vigorous green of the leaves, prior to the appearance of the mould, is not at all an uncommon occurrence: this phenomenon has been noticed in the ears of corn, in which every grain was soon afterwards filled with spores of bunt.

This species was at one time believed to be parasitic on the “white rust,” from which circumstance its specific name was derived. More precise examination proves that it sometimes occurs where no white rust is present, and therefore its parasitism is imaginary. The suckers in this species are large and penetrate the cells, often entirely filling them. This feature is sufficient to distinguish the mycelium of the mould from that of the “rust.” The turnip mould occurs on many cruciferous plants, and especially on the shepherd’s-purse, as well as upon the different varieties of cabbage and its more immediate allies.

Onion Mould.—Another disease, produced by fungi of the same genus, makes its appearance upon young onion plants in the spring. The mould is called Peronospora Schleideniana, and has many features in common with those already described. In this instance the threads are greyish and erect, with alternate branches, not divided by transverse septa, and the spores are obovate, attenuated towards their base (fig. 263). This mould, in some years, is very common and destructive, by preventing the young plants which are attacked from coming to perfection. It is not confined to the onion, but appears on other allied species of Allium (to which the onion belongs). The threads form large patches or blotches on the leaves, and sometimes cover them entirely. It very much resembles the turnip mould, from which the form of the spores considerably differs. This is the same species as that described by Caspary, and afterwards by Berkeley, under the name of P. destructor.

Lettuce Mould.—A very similar mould (Peronospora gangliformis) is sometimes very common in spring on the under surface of the leaves of the cultivated lettuce, appearing in definite white mouldy spots. By reference to the figure of a portion of a thread magnified (fig. 265), it will be seen that the peculiar form of the tips of the branchlets evidences the distinctness of this species.

The oospores are small, globose, and of a yellowish-tawny colour. This mould is by no means confined to lettuces, but has also been found on species of ragwort, sow-thistle, nipplewort, endive, and other composite plants; and has from time to time received numerous names, which it is unnecessary to enumerate.

Tare Mould.—The under surface of the leaves of tares, and sometimes also of peas, is liable to attack from an allied species of mould (Peronospora ViciÆ). In the spring of 1846 it appeared amongst vetches in some districts to such an extent as at one time to threaten the destruction of the crops; but a succession of dry weather at once abridged its power and limited its mischief. Mouldy vetches and mouldy peas are, especially in moist seasons, evils to which the agriculturist knows his crops to be subject; he may not know, however, that this kind of mould (fig. 266) is of so near a kin to that which has acquired such wide-spread fame in connection with the potato. Another species of fungus attacks the garden pea in damp seasons, forming small depressed brownish spots on the leaves and pods; but this is quite distinct from the mould, though probably not less injurious.

The fertile threads are produced in dense clusters, each many times branched, and bearing elliptic acrospores obtuse at their apices, and of a violaceous tint (fig. 266). The oospores are beautifully reticulated and of a yellowish-brown colour (Plate X. fig. 212).

Trefoil and some other allied plants are attacked by another species, characterized by Dr. de Bary as Peronospora trifoliorum, which we have found rather plentifully in some localities on lucern.

The Parsnip Mould (Peronospora nivea, Ung.) is found on many umbelliferous plants; but its attacks upon the parsnip are most to be deplored, because it injures and ultimately destroys an article of human food. The plants infested with this parasite are first attacked in the leaves, but afterwards the roots become spotted and diseased in a similar manner to the potatoes attacked by its congener. The disease has not hitherto been so general with the former as the latter; but in some districts it has been far from uncommon.

The fertile threads are collected in bundles, erect, and not so much branched as in many other species. The acrospores are subglobose or ovoid, and papillate at their apices. This species is sometimes called P. umbelliferarum, and sometimes P. macrospora. Generally speaking the average humidity of a season but little affects the production of parasitic fungi. In a dry season, like that of 1864, we found as many species, and these as flourishing and numerous in individuals, as in a proverbially wet year. Such is not the case, however, with the moulds under notice, or such fungi as are reproduced through the medium of zoospores: these are undoubtedly less common in a very dry season; but it must be remembered that a single shower is sufficient for the development of zoospores, and occasional showers or heavy dews will speed them on their course of destruction as readily almost as continuous moisture. The large fungi, on the contrary, become very limited in numbers when the weather is unusually dry.

Spinach Mould.—Spinach is likewise liable to suffer from the establishment of a mould upon the under surface of the leaves: unfortunately this is not unfrequent, and has been known in England certainly for the last fifty or sixty years, since it was figured by Sowerby in his “British Fungi” as many years since. We have lately seen a bed of spinach, utterly destroyed by this fungus; whilst on another, not twenty yards apart, not a spotted leaf could be found. This mould is the Peronospora effusa of botanists; it occurs also on some species of goosefoot (Plate X. fig. 215), and probably on knotgrass. To the naked eye it appears in pale purplish-grey patches, which, when examined microscopically, are found to consist of dense bundles of branched threads, bearing ellipsoid acrospores, the membranes of which have a violaceous tint. The oogonia produced upon the mycelium vary considerably in size. The oospores are of the character delineated in our plate (Plate X. fig. 214).

Hitherto all the species of mould to which we have had occasion to refer have been found infesting plants more or less employed as food; but there remain one or two other species to which we must make special reference. One of these affects the most universal of favourites amongst flowers: this is the rose mould. Attention was directed to this mould, and it was described for the first time under the name of Peronospora sparsa, in the columns of the Gardeners’ Chronicle, in 1862. It occurred on a quantity of potted rose-plants in a conservatory. Irregular pale brownish discoloured spots appeared on the upper surface of the leaves; these extended rapidly, and in a short time the leaves withered and shrivelled up, and ultimately the whole plant perished. A delicate greyish mould was to be seen by the aid of a lens, scattered over the under surface of the leaves. By the microscope, the branched threads, having the tips furnished with subelliptic spores, were revealed, and an ally of the potato mould found revelling amongst the roses.

During the winter of 1863-4, we found the leaves of several species of dock occupied by a mould which appears to be a very low form of Peronospora. Its presence was indicated by brownish orbicular spots, on which the fertile threads occurred in small bundles. These threads were generally simple, but occasionally forked, bearing rather large elliptical acrospores attached obliquely to the tips of the threads (fig. 269). In consequence of this peculiarity, we have named the species, which does not appear to have been noticed before, Peronospora obliqua. It is clearly very distinct from another species found on dock leaves by Corda.

Of the remaining British species, one (P. ArenariÆ) is found on the leaves of the three-veined sandwort (fig. 268); another attacks the red corn-poppy, a third is found on the common nettle, one on the brooklime, another on the wood-anemone (fig. 267), and another on the figwort.

Doubtless all the species in this genus are possessed of the third means of reproduction, by zoospores, as discovered in the potato mould, not only from the acrospores, but also from the oospores.

The fearful rapidity with which this method enables them to multiply themselves may account for their widely spreading and devastating power. No other genus of fungi can parallel this in the number of species injurious to the field or the garden, or in which the injuries inflicted are so great and irremediable.

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N.B.—Since the foregoing chapter was in type, the Rev. M. J. Berkeley informs us that both Mr. Broome and himself have examined the mould on dock-leaves, to which we have given the name of Peronospora obliqua, and have come to the conclusion that it is truly a member of that genus, and not hitherto described; but they are also of opinion that it is the same mould as one described by Dr. Montagne as Ascomyces Rumicis. We concur with them in thinking it deficient in the important characteristics of Ascomyces, and therefore retain its proposed name of P. obliqua.