Perisporineae.—This includes two chief families, Erysiphaceae
and Perisporiaceae. They are characterized by an ascocarp without
any opening to the exterior, the ascospores being set free by the
decay or rupture of the ascocarp wall; such a fruit-body is termed
a cleistothecium (cleistocarp). The Erysiphaceae are a sharply
marked group of forms which live as parasites. They form a superficial
mycelium on the surface of the plant, the hyphae not usually
penetrating the tissues but merely sending haustoria into the epidermal
cells. Only in rare cases is the mycelium intercellular.
Owing to their appearance they go by the popular name of mildews.
Sphaerotheca Humuli is the well known hop-mildew, Sphaerotheca
Mors-Uvae is the gooseberry mildew, the recent advent of which
has led to special legislation in Great Britain to prevent its spreading,
as when rampant it makes the culture of gooseberries impossible.
Erysiphe, Uncinula and Phyllactinia are other well-known genera.
The form of the fruit body, the difference and the nature of special
outgrowths upon it—the appendages—are characteristic of the
various genera. Besides peritheca the members of the Erysiphaceae
possess conidia borne in simple chains. De Bary brought forward
very strong evidence for the origin of the ascocarp in Sphaerotheca
and Erysiphe by a sexual process, but Harper in 1895 was the first
to prove conclusively, by the observation of the nuclear fusion, that
there was a definite fertilization in Sphaerotheca Humuli by the
fusion of a male (antheridial) nucleus with a female, ascogonial
(oogonial) nucleus. Since then Harper has shown that the same
process occurs in Erysiphe and Phyllactinia.
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| Fig. 11.—Development of Eurotium repens. (After De Bary.)
A, Small portion of mycelium with conidiophore (c), and archicarp (as). B, The spiral archicarp (as), with the antheridium (p). D, The same, beginning to be surrounded by the hyphae forming D, The perithecium. E, F, Sections of young perithecia. w, Parietal cells. f, Pseudo-parenchyma. as, Ascogonium. G, An ascus. H, An ascospore. |
The Perisporiaceae are saprophytic forms, the two chief genera being Aspergillus and Penicillium. The blue-green mould P. crustaceum and the green mould A. herbariorium (= Eurotium herbariorum) are extraordinarily widely distributed, moulds being found on almost any food-material which is exposed to the air. They have characteristic conidiophores bearing numerous conidia, and also cleistothecia which are spherical in form and yellowish in colour. The latter arise from the crown of a spirally coiled archicarp (bearing an ascogonium at its end) and a straight antheridium. Vegetative hyphae then grow up and surround these and enclose them in a continuous sheath of plectenchyma (fig. 11). It has lately been shown by Fraser and Chambers that in Eurotium both ascogonium and antheridium contain a number of nuclei (i.e. are coenogametes), but that the antheridium disorganizes without passing its contents into the ascogonium. There is apparently a reduced sexual process by the fusion of the ascogonial (female) nuclei in pairs. Aspergillus Oryzae plays an important part in saccharifying the starch of rice, maize, &c., by means of the abundant diastase it secretes, and, in symbiosis with a yeast which ferments the sugar formed, has long been used by the Japanese for the preparation of the alcoholic liquor saké. The process has now been successfully introduced into European commerce.
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.
Fig. 12.—Peziza aurantiaca. (After Krombholz, nat. size.) |
Fig. 13.—Ascobolus furfuraceus.
Diagrammatic section of the fructification. (After Janczewski.) m, Mycelium. |
Discomycetes.—Used in its widest sense this includes the Hysteriaceae, Phacidiaceae, Helvellaceae, &c. The group is characterized in general by the possession of an ascocarp which, though usually a completely closed structure during the earlier stages of development, at maturity opens out to form a bowl or saucer-shaped organ, thus completely exposing the layer of asci which forms the hymenium. Such an ascocarp goes by the name of apothecium. Owing to the shape of the fruit-body many of these forms are known as cup-fungi, the cup or apothecium often attaining a large size, sometimes several inches across (fig. 12). Functional male and female organs have been shown to exist in Pyronema and Boudiera; in Lachnea stercorea both ascogonia and antheridia are present, but the antheridium is non-functional, the ascogonial (female) nuclei fusing in pairs; this is also the case in Humaria granulata and Ascobolus furfuraceus, where the antheridium is entirely absent. In H. rutilans, however, both sexual organs are absent and the ascogenous hyphae arise apogamously from the ordinary hyphae of the mycelim. In all these cases the ascogonium and antheridium contain numerous nuclei; they are to be looked upon as gametangia in which there is no differentiation of gametes, and since they act as single gametes they are termed coenogametes. In some forms as in Ascobolus the ascogonium is multicellular, the various cells communicating by pores in the transverse walls (fig. 13).
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From Strasburger’s Lehrbuch der Botanik, by permission of Gustav Fischer.
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| Fig. 14.—Perithecium of Podospora fimiseda in longitudinal section. After v. Tavel.
s, Asci. |
In the Helvellaceae there is no apothecium but a large irregular fruit body which at maturity bears the asci on its surface. The development is only slightly known, but there is some evidence for believing that the fruit-body is closed in its very early stages.
The genus Peziza (in its widest sense) may be taken as the type of the group. Most of them grow on living plants or on dead vegetable remains, very often on fallen wood; a number, however, are found growing on earth which is rich in humus. The genus Sclerotinia may be mentioned here; a number of forms have been investigated by Woronin. The conidia are fragrant and are carried by bees to the stigma of the bilberry; here they germinate with the pollen and the hyphae pass with the pollen tubes down the style; the former infect the ovules and produce sclerotia, therein reducing the fruits to a mummified condition. From the sclerotia later the apothecium develops. One species, S. heteroica, is heteroecious; the ascospores infecting the leaves of Vaccinium uliginosum, while the conidia which then arise infect only Ledum palustre. This is the only case of heteroecism known in the vegetable kingdom outside the Uredineae.
Pyrenomycetes.—This is an extraordinarily large and varied group of forms which mostly live parasitically or saprophytically on vegetable tissue, but a few are parasitic on insect-larvae. The group
