_Cytology of Uredineae._--The study of the nuclear behaviour of the
cells of the Uredineae has thrown great light on the question of
sexuality. This group like the rest of the Basidiales exhibits an
association of nuclei at some point in its life-history, but unlike
the case of the Basidiomycetes the point of association in the
Uredineae is very well defined in all those forms which possess
aecidiospores. We find thus that in the _eu_ and _opsis_ forms the
association of nuclei takes place at the base of the aecidium which
produces the aecidiospores. There we find an association of nuclei
either by the fusion of two similar cells as described by Christmann
or by the migration of the nucleus of a vegetative cell into a special
cell of the aecidium. After this association the nuclei continue in
the conjugate condition so that the aecidiospores, the
uredospore-bearing mycelium, the uredospores and the young
teleutospores all contain two paired nuclei in their cells (fig. 17).
Before the teleutospore reaches maturity the nuclei fuse, and the
uninucleate condition then continues again until aecidium formation.
In the _hemi_, _brachy_, _micro_ and _lepto_ forms, which possess no
aecidium, we find that the association takes place at various points
in the ordinary mycelium but always before the formation of the
uredospores in the _hemi_ and _brachy_ forms, and before the formation
of teleutospores in _micro_ and _lepto_ form. Whether the association
of nuclei in the ordinary mycelium takes place by the migration of a
nucleus from one cell to another or whether two daughter nuclei become
conjugate in one cell, is not yet clear. The most reasonable
interpretation of the spermatia is that they are abortive male cells.
They have never been found to cause infection, and they have not the
characters of conidia; the large size of their nuclei, the reduction
of their cytoplasm and the absence of reserve material and their thin
cell wall all point to their being male gametes. Although in the forms
without aecidia the two generations are not sharply marked off from
one another, we may look up the generation with single nuclei in the
cells as the gametophyte and that with conjugate nuclei as the
sporophyte. The subjoined diagram will indicate the relationship of
the forms.
_Basidiomycetes._--This group is characterized by its greatly reduced
life-history as compared with that of the _eu_ forms among the
Uredineae. All the forms have the same life-history as the _lepto_
forms of that group, so that there is no longer any trace of sexual
organs. There is also a further reduction in that the basidium is not
derived from a teleutospore but is borne directly on the mycelium.
Formerly, before the relationship of promycelium and basidium were
understood, the Uredineae were considered as quite independent of the
Basidiomycetes. Later, however, these Uredineae were placed as a mere
subdivision of the Basidiomycetes. Although the Uredineae clearly lead
on to the Basidiomycetes, yet owing to their retaining in many cases
definite traces of sexual organs they are clearly a more primitive
group. Their marked parasitic habit also separates them off, so that
they are best included with the Basidiomycetes in a larger cohort
which may be called Basidiales. Most of Basidiomycetes are
characterized by the large sporophore on which the basidia with its
basidiospores are borne.
[Illustration: From _Annals of Botany_, by permission of the Clarendon
Press.