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confounded with circumnutation by some authors,* but is simply a hygrometric phenomenon. The distal extremity of the germ-tube of an acidiospore gains an entrance into the tissues of the host-plant through one of the stomata. This is effected by either the end of the germtube or one of its principal branches insinuating itself into the cleft (Plate II. Fig. 10). When an entrance has been achieved, further development takes place in the air-spaces below the stoma, by the branches of the germ-tube growing between the cells of the host-plant; these soon develop into a mycelium, which in due course produces the sporeform proper to the species.

The time which the acidiospores retain their germinative faculty is stated by De Bary † to be some weeks. From my observations, I should say it is rather a matter of hours. Much depends, however, upon the temperature and the amount of dryness to which the spores are exposed. If they be placed in a very moist atmosphere, they germinate at once; if, on the contrary, in a perfectly dry one, they almost as rapidly die. But if they be kept slightly moist and cool, they will remain uninjured for a much longer time. I have, however, seldom found them. germinate after forty-eight hours, and then only a small proportion will do so. It is only the mature spores at the top of the cup, that have already separated themselves from the spore-series, which will germinate. The process begins within a few hours after they have been placed in

water.

Cooke, "Circumnutation in Fungi," Quekett Journal (1884), vol. i. 2nd series, p. 309.

+ De Bary, "Vergleich.," p. 369.

CHAPTER V.

UREDOSPORES.

THE uredospores may be developed from mycelia produced from the entrance of the germ-tube of an acidiospore, a promycelial spore, or another uredospore. The uredospores are developed singly on the ends of separate mycelial hypha (sterigmata, basidia). Their development is as follows:-the mycelial hyphæ in the affected hostplant at certain points become interwoven and intertwined, and the hypha which constitute these favoured points are more richly branched and more freely septate than is the case in other parts of the host-plant. In them may be observed a number of orange granules. The hyphæ are so crowded together in the spore-bed, that to a great extent they lose their individual outline. Spore-formation takes place, first towards the centre of the bed, and then extends centrifugally. The mycelial aggregations take place just beneath the cuticle of the plant, to which their upper surfaces are parallel; they are termed spore-beds, or sori (sporenlager, stroma, hymenia, clinodes). Each sporebed consists of an accumulation of hypha, which ramify in all directions; but those hyphæ that are nearest to the cuticle of the host-plant give off a number of branches parallel to each other, at right angles to the spore-bed below, and the host-plant cuticle above. Each of these

branches is destined to produce a uredospore. At first they are of uniform diameter, but soon the upper end dilates so as to become club-shaped (Plate III. Figs. 2, 5). Into this dilated extremity the protoplasmic contents of the hyphæ are gradually emptied; and thus by their continued accumulation it becomes almost spherical. This dilated tube-end is now full of granular protoplasm, towards the centre of which appear a few orange granules (in the orange-spored species), while externally it becomes invested with a thin cell-wall of its own (Plate III. Fig. 3). The cell increases in size, its walls in thickness, and its contents become more and more distinctly coloured. The orange granules, which consist of reddish-yellow, oleaginous particles, are at first confined to the centre of the cell, and gradually increase in number from the centre outwards; so that we frequently observe immature spores, which are orange only in their centre, having the centre surrounded by a hyaline zone. The spore has at this stage of its development two coats-the outer, which is extremely thin and is the dilated mycelial hypha; and the inner, the proper wall of the spore. The latter increases somewhat in thickness, although it always constitutes a comparatively thin investment. These two coatings become closely applied to one another. The exospore, which is at first smooth, becomes, in most species at their maturity, variously roughened by the appearance of minute projections from its surface (Plate III. Fig. 4). These may be in the form of short, fine spines, when the spore is said to be finely echinulate, or the prominences may be shorter and less acutely pointed (verrucose). De Bary has shown that the spores of many of the Uredinea owe their roughness to minute, densely crowded, prismatic, staff-like processes, between which similar smaller processes are closely packed. The irregularities of the exospore disappear under

the action of caustic potash; and they are much less easily observed when the spores are examined in water than when they are seen dry. This is equally true of the æcidiospores.

In the endospore are two or more openings (germpores, oscules) through which the germ-tubes emerge when the spores germinate. They can sometimes be made out pretty clearly in perfectly ripe spores, but not often. There is no difficulty, however, in observing them in those spores in which the process of germination has commenced. In the globose uredospores they are arranged in a circle round the equator; in the oval spores, also midway between the poles. Whether it be correct to regard them as openings is doubtful; they would be more correctly described as thin places, which become holes. The number of germ-pores varies in different species; they are never less than two. Their variation in number and position is but slight thus De Bary* gives for U. linearis, four; U. fabæ,† three; U. phaseoli,‡ two; and U. suaveolens,§ three.

Recently De Bary || has stated that in the uredospores of Puccinia and Uromyces the germ-pores are sharply defined, circular holes in the endospore, closed externally by the exospore; but this hardly accords with my observations. The spore-bed continues to produce uredospores for some considerable time; at length it ceases to do so. If it be examined in this condition it will be found to consist of little else than barren basidia, with here and there one bearing a spore. The uredospores vary in colour; most are some shade of orange, many are brown. De Bary¶ has pointed out that in those species with brown spores the contents are colourless, e.g. in U. phaseoli, rumicis,

* De Bary, "Brandpilze," p. 33. Ibid., p. 76.

|| De Bary, "Vegl.," p. 109.

De Bary, "Champ. paras.," p. 74. § De Bary, "Brandpilze," p. 33. ¶ De Bary, "Brandpilze," p. 31.

trifolii, etc., the brown colour being due to the spore-wall. The colouring matter of the uredo and æcidiospores has been spectroscopically investigated by Bachmann.* He examined the acidiospores of Gymnosporangium juniperinum and Puccinia coronata, and the uredospores of Melampsora farinosa, Triphragmium ulmaria, and Uromyces alchemillæ. Combined as it is with some oleaginous material in granules, and enclosed within the cell walls of the spores, it is exceedingly difficult to extract it with ether. But by adopting the saponification process of Kühne† and Hansen ‡ he was able to arrive at the following conclusions. The above-named fungi give very similar spectra, namely two narrow absorption bands, one between band F, the other between F and G, showing that the pigment is the same in all cases; that it is very similar to, if not identical with, the colouring matter of most of the yellow phanerogamous blossoms (the anthoxanthin of Hansen) although combined differently, and allied to the xanthophyll group. It is soluble in alcohol, ether, chloroform, bisulphide of carbon, and benzol, and is coloured green by potassium iodide.

As soon as the uredospore has arrived at its maturity. it becomes separated below from the hypha which produced it (basidium, sterigma). This separation takes place by the basidia breaking off either close to the spore (Uredo) or at a short distance below, in which case the upper part of the basidium remains attached to the spore (Trichobasis). This separation is facilitated, if not caused, by the spore being pushed off by the continued formation of other

* E. Bachmann, "Spektroskopische Untersuchungen von Pilzfarbstoffen " (1886), pp. 21-23, figs. 27-31.

† W. Kühne, "Ueber lichtbestandige Farben der Netzhaut" ("Untersuch. d. physiol. Inst. d. Univ. Heidelberg "), bd. i. hft. iv. p. 347.

F. A. Hansen, "Der Chlorophyll Farbstoff” (“ Arbeiten des botan Inst. zu Würzburg"), bd. iii. hft. i. p. 126.

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