Part 29

To this series belong the lowest of the Basidiomycetes. The basidia appear in two principal forms (1 and 2 on page 144) and are divided into four cells, either transversely or longitudinally, each division forming a sterigma which abstricts a basidiospore. The first three orders, Uredinaceæ, Auriculariaceæ, and Tremellaceæ have gymnocarpic fruit-bodies, while those of the Pilacraceæ, on the contrary, are angiocarpic.

Order 1. =Uredinaceæ (Rusts).= All the Rust-Fungi are parasites, their mycelium living in the interior of the stems and leaves of their hosts, causing red, brown, or black spots--hence their name--and malformations, sometimes of considerable size.

The Rust-Fungi are gymnocarpic and destitute of a hymenium; for these reasons they are regarded as the simplest order of the Basidiomycetes. They are entirely parasitic, and their filamentous, branched mycelium ramifies in the intercellular spaces of its host, and often protrudes haustoria into the cells. The mycelium is perennial should it enter a woody tissue; it may also hibernate in the rhizomes of perennial herbs and permeate the shoots springing from them, but in the majority of the Rust-Fungi the mycelium has a very limited growth. The chief means of reproduction of the Rust-Fungi are the chlamydospores, which in the more highly developed species occur in three forms, namely, the teleuto-, æcidio-, and uredo-spores. The spores, in the host, are formed immediately beneath its epidermis, which is ruptured on the ripening of the spores, with the production of “rust,” brown, red, or black spots. Those chlamydospores which produce basidia are termed teleutospores. The spore on germination produces a transversely divided basidium, “promycelium,” on which basidiospores, “sporidia,” generally four in number, are produced on lateral sterigmata. This basidio-fructification is gymnocarpic; the basidia neither form a hymenium nor a fruit-body (only Cronartium and Gymnosporangium have a slight indication of a basidio-fructification).

Many Rust-Fungi, in addition to basidiospores, have small, unicellular conidia, “spermatia,” which are borne in conidiocarps, “spermogonia.”

The ~TELEUTOSPORES~ (Winter-spores) may be either unicellular or multicellular; in the majority of cases they are enclosed in a hard outer cell-wall, the exospore, which in some cases is very strongly developed; they have also a long or short stalk, the remains of the spore-bearing hypha. Each cell of the teleutospore has one germ-pore (a thin portion of the wall, for the protrusion of the germ-tube; in Phragmidium and Gymnosporangium there are, however, several germ-pores). The colour of the teleutospores is generally much darker than that of the uredospores, and it is by these that the majority of the Rust-Fungi hibernate.

In Gymnosporangium, two kinds of teleutospores are found (distinguished by their size and thickness of exospore). In many species of Puccinia, the form of the teleutospores varies very much, so that in the same layer spores have been observed with the characteristic form of other, allied genera.--The teleutospores of Endophyllum resemble æcidiospores, since they are united in chains, whose cells are easily separated, and are produced in the interior of a “peridium.” The multicellular teleutospores of Coleosporium function as basidia, and from each cell immediately produce basidiospores.--The teleutospores of Coleosporium and Chrysomyxa, differ from other teleutospores in the absence of exospore and germ-pore.

The ÆCIDOSPORES (Spring-spores) are produced in chains which are generally enclosed in an envelope of hyphæ, the peridium; the peridium enclosing the spores being termed the æcidium. The æcidiospores are unicellular, and generally of an orange colour; they are often separated by intermediate cells which wither and so assist in the distribution of the spores. The exospore is made up of minute, radially arranged rods. Generally germination proceeds immediately, the æcidiospore producing a germ-tube, which developes into a mycelium bearing either uredo- or teleutospores.

The æcidia of many Rust-Fungi were formerly considered as distinct genera. The æcidia of Phragmidium, Triphragmium, and Melampsora, in which the peridium is wanting, were in part considered as Cæoma. The æcidia with fimbriate edge, or those of Gymnosporangium with longitudinal lattice-like splits, were considered as “Rœstelia” (Lattice-Rust); large, sac-shaped æcidia on the Coniferæ were known as Peridermium.

The ~UREDOSPORES~ (Summer-spores) are unicellular and arise singly, seldom in chains (Coleosporium). Their colourless, warty exospore bears, in the equatorial plane, 2–8 germ-pores. In the majority, germination proceeds immediately, and a mycelium is produced which at first gives rise to uredospores and afterwards to teleutospores.

The uredospore-formations of Melampsorella and Cronartium are enclosed in an envelope, and hence resemble æcidia.--Between the uredospores sterile, unicellular hyphæ (paraphyses) may be found.

The spermogonia are spherical or pear-shaped conidiocarps, generally embedded in the substratum, and are produced before the æcidia, before or simultaneously with the uredospores, or before the teleutospores. The conidia, as far as observations go, do not generally germinate under ordinary conditions.

Among the Rust-Fungi some species are found which only form basidiospores and teleutospores (Puccinia malvacearum, Chrysomyxa abietis). Other species have in addition uredospores; others spermogonia and uredospores; others spermogonia and æcidia; others spermogonia, uredospores and æcidia. Those species in which all the methods of reproduction are not developed must not be considered as incomplete forms.

As a rule the mycelium, which is produced from the basidiospores, developes æcidia; in the species, however, without æcidia, it developes the uredo-form, and when the uredospores are also absent, the teleutospore-form. It has been established in some species of Puccinia and Uromyces that the formation of æcidia can be suppressed, and it is not a necessary part of the cycle of development of the species.

The majority of Rust-Fungi hibernate in the teleutospore-form. Many species are able to hibernate in the uredospore-form (Coleosporium senecionis). Others pass the winter in the æcidio-form, and develope æcidia on new hosts (Uromyces pisi, on Euphorbia cyparissias; Phragmidium subcorticium, on Rosa; Æcidium elatinum, on Abies alba). In Chrysomyxa abietis, the mycelium, developed from the basidiospores, survives the winter.

Among the Rust-Fungi, with several forms of reproduction, there are about sixty whose development can only be completed by an alternation of hosts, that is, on one host only uredo-and teleutospores are produced, while the further development of the germinating basidiospores, and the formation of the æcidia and spermogonia from its mycelium, can only take place on a second quite distinct and definite host (heterœcious or metoxenous Fungi). Those Fungi which have all their forms of reproduction on the same host are termed autœcious or autoxenous. It is not, however, always necessary that the heterœcious Rust-Fungi should regularly change their hosts; for example, Puccinia graminis can hibernate in the uredo-form on the wild Grasses, and in the spring can distribute itself again in the same form.

As a consequence of the alternation of hosts the various forms of development were considered as independent genera (Uredo, Æcidium, Rœstelia, Cæoma, Peridermium), until De Bary and Oersted established, about the same time (1865), the mutual connection of some forms, and paved the way for the right conception of these Fungi.

As an example of one of the most highly developed species, Puccinia graminis, the “Rust of Wheat,” holds a prominent position. Its uredospores and teleutospores are produced (Fig. 146) on Grasses (on cereals, especially Wheat, Rye, Oats, and many wild Grasses), while the æcidia and spermogonia are confined to the Berberidaceæ. The teleutospores, developed on the Grasses, hibernate on the dried portions of their host, and in the succeeding year each of the two cells of the teleutospore may develop a basidium with four basidiospores (Fig. 146 D, c). The basidiospores are distributed by the wind, germinate quickly, and only proceed to further development on Berberis or Mahonia. The germ-tube bores through the epidermis of the Barberry-leaf, and forms a mycelium in its interior, its presence being indicated by reddish-yellow spots on the leaf. After 6–10 days the flask-shaped spermogonia appear (Fig. 147 B; C, a; conidia in Fig. 147 D) and a few days later the cup-shaped æcidia (Fig. 147 A; C, c, d, e). The former are generally on the upper, and the latter on the under side of the leaf. The orange-coloured æcidiospores scatter like dust, and germinate only on Grasses; the germination takes place in about two days when placed on any green part of a Grass. The germ-tube enters the Grass-leaf through a stoma; a mycelium is developed in the leaf, giving rise to a small, oval, rust-coloured spot (Fig. 146 A); in about 6–9 days the epidermis is ruptured over the red spot, and numerous reddish-yellow uredospores, formed on the mycelium, are set free. The uredospores (Fig. 146 B) are scattered by the wind, and can germinate should they fall on the green portions of other Grasses: they then emit 2–4 germ-tubes through the equatorially-placed germ-pores. The germ-tubes enter a leaf through a stoma, a new mycelium is then developed, and in about eight days a fresh production of uredospores takes place, which germinate as before. The uredospore-mycelium very soon produces, in addition, the brown teleutospores, which give a brown colour to the rust-coloured spots, the familiar uredospores on the cereals being quite suppressed towards the close of the summer (Fig. 146 C, D). The “Rust of Wheat” hibernates on some wild Grasses in the uredospore-form.

GENERA. Puccinia (Fig. 146, 147) has bicellular teleutospores, each having a germ-pore, and the æcidia when present have an indented peridium; some species, as exceptions, have 1–3-celled teleutospores. Many species are HETERŒCIOUS, for example, P. graminis, described above; P. rubigo, which also infests various Grasses, but whose æcidia appear on Anchusa; the masses of teleutospores are small; they contain paraphyses, and are for a long time covered by the epidermis. P. coronata, on Oats and Rye Grass; its æcidia on Rhamnus; the teleutospores are surmounted by a crown--“coronate processes.” P. phragmitis, on Reeds; æcidia on species of Rumex and Rheum. P. moliniæ, on Molinia cœrulea; the æcidia on Orchids. P. poarum, on Meadow-Grass; æcidia on Tussilago. Various Puccinias growing on species of Carex have their æcidia on Urtica, Lysimachia, Cirsium, Pedicularis, etc.--Of those AUTŒCIOUS species, which have all their generations on the same host, may be noted:--P. galii, P. menthæ, P. violæ, P. epilobii, P. asparagi, which grow on the hosts from which they have taken their specific names.--As representative of a group which have spermogonia, uredo-and teleutospores on the same host, but on different individuals, P. suaveolens, on the Field-Thistle, may be mentioned. The spermogonia have a strong odour.--A peculiar group (Leptopuccinia) has only teleutospores, which germinate immediately, and whilst still attached to their living host. To this group belong P. arenariæ, on a number of Caryophyllaceæ; and P. malvacearum, on various Malvaceæ, introduced in 1873 from South America to Europe, where it soon proved very destructive to Hollyhocks.

Uromyces (Fig. 149) differs only from Puccinia in always having unicellular teleutospores. Among this genus both heterœcious and autœcious species are found. To the first group belong U. pisi, whose æcidia are found on Euphorbia cyparissias, and U. dactylidis, whose æcidia appear on Ranunculus; to the second group belong U. betæ, U. phaseoli, U. trifolii.

Triphragmium has teleutospores with three cells (one below and two above), on Spiræa ulmaria.

Phragmidium (Fig. 150) has teleutospores consisting of a row of cells (3–10) arranged in a straight line; the upper cell has one germ-pore and the others four germ-pores placed equatorially. Both this and the preceding genus have large, irregular æcidia without peridia, but often with bent, club-like paraphyses (150 b and c); they are all autœcious, and are only found on the Rosaceæ.

Endophyllum (see above, under teleutospores, p. 147) on species of Sempervivum.

Gymnosporangium (Figs. 152, 154) has bicellular teleutospores collected in large, gelatinous masses formed by the swelling of the long spore-stalks; in each cell 2–4 germ-pores are found. Uredospores are wanting. All the species are heterœcious; the teleutospores appear on Juniperus, the æcidia (Rœstelia) on the Pomaceæ. G. sabinæ, on Juniperus sabina, J. virginiana, etc., has the æcidia (“Rœstelia cancellata”) on Pyrus communis (Figs. 152, 148); G. juniperinum, on Juniperus communis with “Rœstelia cornuta” (Fig. 154 a) on Sorbus aucuparia, Aria nivea (S. aria) and Malus communis; G. clavariæforme on Juniperus communis, the æcidium belonging to it (“Rœstelia lacerata”) on Cratægus oxyacantha.

Melampsora has prismatic teleutospores placed parallel to each other and forming a crustaceous layer; in many species they are divided longitudinally into several cells (Fig. 151). The æcidia, without peridium, belonged to the old genus Cæoma. M. caprearum, on Willows, has the æcidia (Cæoma euonymi) on Euonymus. M. hartigii, on Osiers; the æcidium on Ribes. M. mixta, on Salix repens and Orchids. M. pinitorqua, on leaves of the Aspen, æcidia on Pine branches (Pine shoot fungus); M. populina on Populus monilifera and nigra; M. betulina (Fig. 153), on Birch leaves; M. padi (Fig. 151), on leaves of Prunus padus, developes teleutospores in the epidermal cells; M. lini is the cause of injury to the Flax; M. agrimoniæ.

Calyptospora gœppertiana; teleutospores on Vaccinium vitis idæa; spermogonia and æcidia on Abies alba (Firneedle-Rust).

Coleosporium (Fig. 155) forms its uredospores in reddish-yellow chains; for the teleutospores, see page 147. C. senecionis, on the Groundsel; its æcidium (Peridermium wolffii) on Pine-leaves (Fig. 155 a). Other species on Sonchus, Petasites, Campanula, Rhinanthaceæ.

Chrysomyxa (Fig. 156) has bright red, branched teleutospore-chains; each spore developes a 4-celled basidium. C. ledi, on Ledum palustre; its æcidia on the leaves of the Fir. C. abietis (Fig. 156), without uredo-and æcidiospores; teleutospores on the leaves of the Fir. In the first summer, yellow bands are formed on the leaves, and in the following spring the red cushions of spores.

Cronartium (Figs. 157, 159) has unicellular teleutospores united in numbers to form erect threads or columns; the uredospores are enclosed in a “peridium”; C. ribicola (Fig. 157), on leaves of Ribes (especially Black Currants); its æcidia (Peridermium strobi, or P. klebahni) on the stems and branches of Pinus strobus (Fig. 159), on which it causes great damage; C. asclepiadeum, on Vincetoxicum officinale; its æcidia (Peridermium cornui) on the stems and branches of Pinus silvestris.