33^ 



THALLOPHVTES. 



spermogonia, the significance of which in both cases is obscure. They make 

 their appearance usually just before the aecidium-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 ^cidiomycetes 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 iEcidiomycetes 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 Ailcidium 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 EuphorbicB cyparissiai^ 

 Endophyllum Sempewi'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, jEcidium Leguminosarum and Trago- 

 pogonls : in others the various reproductive forms are developed upon diflferent hosts, 

 for example, the aecidium-fruits of JEcidium Berberidis occur only on the leaves of 

 Berberis ruulgaris, whilst 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 oi Juniper us. 

 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 oft' 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 Uromyces, 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 A^cidium 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, ji 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 A^cidia. T*he 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- 



