33^ 
THALLOPHYTES. 
spermogonia, the significance of which in both cases is obscure. They make 
their appearance usually just before the secidium-fruits and close beside them upon 
the leaves of the host. They extrude small conidia (spermatia) formed by abstric- 
tion, the significance of which in the life-history of the --iEcidiomycetes is completely 
unknown. It has been suggested that they are to be regarded as parasites, although 
the extraordinary constancy of their appearance certainly militates against this view. 
The ^cidiomycetes exclusively inhabit living Phanerogams, mostly their stems and 
leaves, but also the living cortex of trees (Goniferae). The extension of the mycelium 
in the intercellular passages of the host does not usually produce much injury, though in 
some cases the host becomes deformed, as for instance, the formation of 'witches- 
brooms ' in Firs by the growth of ^cidium elatinum. Occasionally the mycelium is 
confined to certain circumscribed areas of its host {^cidium Leguminosarum and others), 
but more commonly it extends throughout its tissues {JEcidium Euphorbien cyparissise, 
Endophyllum Semper'vi'vi). The fructifications, as also the conidia, are developed beneath 
the epidermis of the host, and penetrate through it to the surface only when they are 
mature. 
Some of the best-known forms possessing conidia avail themselves of the same p^.ant 
as a host throughout their whole life ; for example, ALcidium Leguminosarum and Trago- 
pogonis : in others the various reproductive forms are developed upon different hosts, 
for example, the aecidium-fruits of ^cidium Berberidis occur only on the leaves of 
Berberis njulgaris, v^hilst the uredospores and the teleutospores are formed only 
upon Grasses. Similarly the large aecidium-fruits of Roestelia cancellata occur only 
upon the leaves of Pomaceae, the teleutospores only upon those of species of Juniperus. 
Such forms as these are said to be heteroecious (metoecious), to distinguish them from 
those above-mentioned which inhabit the same host throughout their whole life 
(autoecious). 
The sporidia developed from the promycelium (whether this is derived from aecidio- 
spores or from teleutospores) give off hyphae which pierce the walls of the epidermal 
cells and penetrate into the interior of the host, whilst the hyphae derived from the 
aecidiospores and from the uredospores grow upon the epidermis of the host until 
they reach a stoma through which they enter the intercellular passages. To this rule 
Puccinia Dianthi offers an exception, in that the promycelium derived from the teleuto- 
spores forms sporidia which send their hyphae through the stomata. 
Both the uredospores and the teleutospores protrude their hyphae from certain 
definite portions of their surface at which the cuticularised external membrane (the 
exospore) is either wanting or is very thin. Each uredospore presents from three to 
six such areas lying in its equator, and each teleutospore has one in each cell. The 
teleutospores may be single, as in Uromjces, or in pairs, as in Puccinia, or three may be 
aggregated together, as in Triphragmium, or even four, as in Phragmidium. They 
germinate, usually after a considerable period of rest, in the spring, but occasionally 
immediately after their formation [Roestelia, Puccinia Dianthi). 
In order to illustrate their life-history, I select as an example the Fungus the 
uredospores of which produce the 'rust' of Wheat, the yEcidium Berberidis, hitherto 
known as Puccinia Graminis. 
On the leaves of Berberis 'vulgaris are found in the spring yellowish swollen spots, 
where dense masses of mycelial filaments are interposed between the parenchyma-cells 
(Fig. 221, A and I, the felted mycelium, lying between the cells, being indicated by 
dots). In these swollen spots are found two kinds of fructification, the Spermogonia, 
which are produced somewhat earlier, and the yEcidia. The spermogonia (Fig. 223, /, sp) 
are urn-shaped receptacles surrounded by a layer of mycelium as an envelope; hair-like 
threads which clothe the cavity protrude in the form of a brush from the opening of 
the spermogonium, penetrating the epidermis of the leaf ; the bottom of the spermo- 
