_Basidiales._--This very large group of plants is characterized by the
possession of a special type of conidiophore--the basidium, which
gives its name to the group. The basidium is a unicellular or
multicellular structure from which four basidiospores arise as
outgrowths; it starts as a binucleate structure, but soon, like the
ascus, becomes uninucleate by the fusion of the two nuclei. Then two
successive nuclear divisions occur resulting in the formation of four
nuclei which later migrate respectively into the four basidiospores
(fig. 15). The Basidiales are further characterized by the complete
loss of normal sexuality, but at some time or other in the
life-history there takes place an association of two nuclei in a cell;
the two nuclei are derived from separate cells or possibly in some
cases are sister nuclei of the same cell. The two nuclei when once
associated are termed "conjugate" nuclei, and they always divide at
the same time, a half of each passing into each cell. This conjugate
condition is finally brought to a close by the nuclear fusion in the
basidium. Between the nuclear association and the nuclear fusion in
the basidium many thousands of cell generations may be intercalated.
This nuclear association of equivalent nuclei apparently represents a
reduced sexual process (like the fusion of female nuclei in _Humaria
granulata_ and of vegetative nuclei in _H. rutilans_, among the
Ascomycetes) in which, however, the actual fusion (normally, in a
sexual process, occurring immediately after association) is delayed
until the formation of the basidium. During the tetrad division in the
basidium nuclear reduction occurs. There is thus in all the Basidiales
an alternation of generations, obscured, however, by the apogamous
transition from the gametophyte to sporophyte. The sporophyte may be
considered to begin at the stage of nuclear association and end with
the nuclear reduction in the basidium.
[Illustration: FIG. 16.--_Puccinia graminis._
A, Mass of teleutospores (t) on a leaf of couch-grass.
e, Epidermis ruptured.
b, Sub-epidermal fibres. (After De Bary.)
B, Part of vertical section through leaf of Berberis vulgaris, with
a, aecidium fruits, p, peridium, and sp, spermogonia. (After Sachs.)
C, Mass of uredospores (ur), with one teleutospore (t).
sh, Sub-hymenial hyphae. (After De Bary.)]
_Uredineae._--This is a large group of about 2000 forms. They are all
intercellular parasites living mostly on the leaves of higher plants.
Owing to the presence of oily globules of an orange-yellow or
rusty-red colour in their hyphae and spores they are termed
Rust-Fungi. They are distinguished from the other fungi and the rest
of the Basidiales by the great variety of the spores and the great
elaboration of the life-history to be found in many cases. Five
different kinds of spores may be present--teleutospores, sporidia ( =
basidiospores), aecidiospores, spermatia and uredospores (fig. 16).
The teleutospore, with the sporidia which arise from it, is always
present, and the division into genera is based chiefly on its
characters. The teleutospore puts forth on germination a four-celled
structure, the promycelium or basidium, and this bears later four
sporidia or basidiospores, one on each cell. When the sporidia infect
a plant the mycelium so produced gives origin to aecidiospores and
spermatia; the aecidiospores on infection produce a mycelium which
bears uredospores and later teleutospores. This is the life-history of
the most complicated forms, of the so-called _eu_ forms. In the
_opsis_ forms the uredospores are absent, the mycelium from the
aecidiospores producing directly the teleutospores. In _brachy_ and
_hemi_ the aecidiospores are absent, the mycelium from the sporidia
giving origin directly to the uredospores; the former possess
spermatia, in the latter they are absent. In _lepto_ and _micro_ forms
both aecidiospores and uredospores are absent, the sporidia producing
a mycelium which gives rise directly to teleutospores; in the _lepto_
forms the teleutospores can germinate directly, in the _micro_ forms
only after a period of rest. We have thus a series showing a
progressive reduction in the complexity of the life-history, the
_lepto_ and _micro_ forms having a life-history like that of the
Basidiomycetes. The _eu_ and _opsis_ forms may exhibit the remarkable
phenomenon of heteroecism, i.e. the dependence of the fungus on two
distinct host-plants for the completion of the life-history.
Heteroecism is very common in this group and is now known in over one
hundred and fifty species. In all cases of heteroecism the sporidia
infect one host leading to the production of aecidiospores and
spermatia (if present), while the aecidiospores are only able to
infect another host on which the uredospores (if present) and the
teleutospores are developed. A few examples are appended:
+-----------------------------+------------------+------------------+
| Species. | Teleutospores on | Aecidiospores on |
+-----------------------------+------------------+------------------+
| _Coleosporium Senecionis_ | _Pinus_ | _Senecio_ |
| _Melampsora Rostrupi_ | _Populus_ | _Mecurialis_ |
| _Pucciniastrum Goeppertiana_| _Vaccinium_ | _Abies_ |
| _Gymnosporangium Sabinae_ | _Juniperus_ | _Pyrus_ |
| _Uromyces Pisi_ | _Pisum, &c._ | _Euphorbia_ |
| _Puccinia graminis_ | _Triticum, &c._ | _Berberis_ |
| _P. dispersa_ | _Secale, &c._ | _Anchusa_ |
| _P. coronata_ | _Agrostis_ | _Rhamnus_ |
| _P. Ari-Phalaridis_ | _Phalaris_ | _Arum_ |
| _P. Caricis_ | _Carex_ | _Urtica_ |
| _Cronartium Ribicola_ | _Ribes_ | _Pinus_ |
| _Chrysomyxa Rhododendri_ | _Rhododendron_ | _Picea_ |
+-----------------------------+------------------+------------------+
Some of the Uredineae also exhibit the peculiarity of the development
of biologic forms within a single morphological species, sometimes
termed specialization of parasitism; this will be dealt with later
under the section Physiology.
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by
permission of Gustav Fischer.