Group of spore-cases (sorus) on back of leaf (X 4).
Among the simplest cases are the sheet-like aggregates of sporogenous hyphae in Puccinia, Uromyces, &c., or of basidia in Exobasidium, Corticium, &c., or of asci in Exoascus, Ascocorticium, &c. In the former, where the layer is small, it is often termed a sorus, but where, as in the latter, the sporogenous layer is extensive, and spread out more or less sheet-like on the supporting tissues, it is more frequently termed a hymenium.
P. Dictyota dichotoma, a sorus of oogonia.
Dictyota dichotoma, part of a sorus of antheridia.
(All except d represent vertical sections of sporangiophore or sorus.) canal the spermatozoid, which in the Ferns has been shown to be attracted by reason of its positive irritability to malic acid, passes and fuses with the ovum.
The form of the sorus and the structure of the sporangium are of great systematic importance.
The sorus is frequently protected by an outgrowth from the surface or margin of the leaf called the indusium.
The sporangia arise simultaneously in the sorus, which is borne on the under surface of the ordinary pinna; in those species with large sporangia the latter form a single circle, in others sporangia may also arise from the central part of the receptacle.
The sorus has a somewhat elongated receptacle, on which the sporangia arise basipetally; the indusium may be cup-shaped, bivalve or wanting.
A consideration of the Filicaceae as arranged above will show that the several sub-orders may in general terms be said to form a series between those in which the sorus consists of a single circle of bulky sporangia and those Polypodiaceae in which the numerous small sporangia appear to be grouped without order in the sorus.
The extinct forms cannot be dealt with in detail here; but it may be pointed out that their order of appearance affords a certain amount of direct evidence that the existing Ferns with a single circle of large sporangia in the sorus are relatively primitive.
The series which can be constructed from a study of the sorus is in general supported by the anatomy of the sporophyte, and by the structure and sexual organs of the gametophyte.
On the ground mainly of an examination of the sorus and sporangium, Bower has shown that the Filicaceae may be divided into three groups - the Simplices, Gradatae and Mixtae - in which the sporangia arise simultaneously, in basipetal succession, or irregularly in the sorus respectively.
The change from the one type of sorus to the other may have taken place in several different lines of descent, some of which have been traced.
A consideration of the biology of the sorus gives an insight into the advantages obtained by the one type over the preceding, as regards protection, spore production and the dispersal of the spores, and thus indicates the way in which natural selection may have acted.
The differences in the form and mode of dehiscence of the sporangia (those of the Simplices having median dehiscence and a horizontal annulus, those of the Gradatae a more or less oblique position of the annulus and of the plane of dehiscence, while in the Mixtae the annulus is vertical and the dehiscence transverse) stand in relation to the position of the sporangia in the sorus relatively to one another.
Each sorus includes both microsporangia, with numerous spores, and megasporangia, each of which contains a single megaspore with a complicated wall.
These coloured spots are due to the presence of a sorus or layer of countless numbers of minute brown spores, the uredospores of the summer fruiting form.
The fine thread-like filaments composing the mycelium of the fungus are embedded in the tissue underneath and around the uredo-sorus, and draw from the host the nourishment required.
Infection of the leaves of the young wheat plants follows on the scattering of the aecidiospores: a sorus of the rusty uredospores is produced, and the life-cycle is complete.
Sorus in surface-view, magnified.