Uredineæ. These permeating mycelia, whether of the Uredinea or Ustilagineæ, are perennial. When the plant, if perennial, dies down in winter, the mycelium, of course, dies down with it, but remains alive, although quiescent, in the upper part of the root-stock; and when fresh shoots are sent up in spring, the mycelium is sent up in them. One peculiarity of most of the Ustilagineous mycelia is that, although it pervades more or less the whole plant, it produces its spore-formation at certain favoured places only; these are, for the most part, in the flowers or seeds. of the plants, but not always, sometimes on the stems or in the leaves. The place of spore-formation, however, is constant with each species. If a plant affected with one of the Ustilagineæ be transplanted into a garden, it will, year after year, be affected with the parasite. In my own garden at King's Lynn I have had growing for the past six years, plants of Colchicum autumnale affected with Urocystis colchici, Triticum repens with U. hypodytes, and Avena clatior with U. segetum. De Bary mentions that a plant of Saponaria officinalis, in the Freiburg Botanic Garden, was for more than ten successive years affected with U. violacea.

CHAPTER IX.

FORMATION OF THE TELEUTOSPORES OF THE

USTILAGINEÆ.

WHILE it is true that the spores of Ustilagineæ are formed from the mycelium, yet the process does not take place directly from the vegetative mycelium which has just been described. On the contrary, at those favoured parts of the affected host-plant at which the spores are developed the vegetative mycelium often quite suddenly changes its character. The double-contoured hyphæ with pellucid vacuolate contents lose their double contour, become swollen or distended, and contain, instead of a clear watery fluid, a gelatinous, granular protoplasm in which numerous oleaginous particles may often be seen (Plate V. Figs. 3–6). The gelatinization of these spore-forming hyphæ is a great character of the Ustilagineæ; it does not, however, occur in all species. The first change observable in the mycelium before it becomes a spore-forming hypha, is that its walls. increase in thickness at the expense of its calibre, which becomes proportionately diminished; soon, however, the whole hypha becomes dilated, so that its lumen is increased. Its contents can now, by the action of reagents, be shown to consist of protoplasm. Spore-formation takes place, after these changes in the mycelium, so differently in the different genera that it will be necessary to describe the process, as it takes place in each one, separately.

Ustilago. The spore-forming hyphæ enlarge and branch in various ways. The gelatinization of their interior takes place to such an extent as to almost obliterate their lumen, which, however, may frequently be seen as a narrow shining line in the middle of the hypha (Plate V. Fig. 7). At certain points the surface of these hyphæ enlarge, so that they appear nodose. The increase in size of the hypha continues, so that adjacent hyphæ become variously tangled and intertwined together, and eventually many of the hyphæ appear glued together, or to have coalesced. The irregularities of the hyphæ become more marked, and it is obvious that each tumefaction will eventually become a spore, inasmuch as they gradually get more and more rounded (Plate V. Fig. 8). It is in the interior of these distended hyphæ that spore-formation takes place. It is, however, always found that the external spores are the most developed, the formation being, in fact, centripetal. The commencement of the differentiation of the protoplasmic contents is at the exterior of the mass, and it gradually proceeds inward towards the centre. The spores when first formed have gelatinous envelopes, and gradually become more or less polygonal from mutual compression. The interior of the spore is now seen to have a distinct contour, and to contain fatty granules. The outer edge of this contour darkens, and even while it is still surrounded by a thin gelatinous envelope the irregularities of the epispore begin to be apparent. As the spores ripen this gelatinous membrane disappears, so that at their maturity they have no remnant of it; nor are any remains of the mycelial hyphæ attached to them, as is often seen in Tilletia.

Sphacelotheca.-This genus differs from Ustilago in the spore-mass being developed in a receptacle. De Bary thus describes its development: "The vegetative mycelium,

1

entering the ovary through the flower-stalk, sends its hyphæ through the funiculus into the ovule, which becomes permeated by densely interwoven hypha. The micropylar end of the integuments alone escapes and remains, as a cap on the top of the diseased ovule, for some time,

Fig. 4-Sphacelotheca hydropiperis (Schum.) A, Ovary and perianth of Polygonum hydropiper affected with S. hydropiperis. B, The same more advanced, showing the micropylar cap (c). C, Section of ovary and perianth at an early stage; g, the style; o, the integument of the micropylar end of the ovule; f, wall of the ovary; p, the perianth. The spore-formation is seen to be commencing above, and the rudimentary columella is visible. D, Section of a more advanced ovary, showing the walls of the receptacle and the columella (c). Slightly magnified. (De Bary.)

but eventually falls off. The hypha develop partly into spores and partly into the receptacle. The latter consists of a thick external case with a central columella. The cells of which it is composed are but loosely compacted, colourless, and about the size of the spores. The least injury fractures this case and allows the escape of the

spores." The spore-formation itself is similar to that of

*

Ustilago, and, commencing above, proceeds downwards. Sorosporium. The spore-formation in Sorosporium differs considerably from that which has just been described in Ustilago, although it is obviously of the same type. In Ustilago the mature spores are separate and distinct, forming usually a pulverulent mass. In Sorosporium, on the other hand, they are in their perfect state aggregated into spore-balls, which individually often contain fifty or a hundred separate spores. The process of spore-formation has been studied by Von Waldheim with S. saponaria, and is as follows:-The mycelium, which is very abundant in the blossom and ovary, rapidly changes into spore-forming hypha, from 4 to 7u in diameter, which are gelatinous and full of shining protoplasm. The free ends of these hyphæ have a tendency to curve inwards and roll themselves up (Plate V. Fig. 9). The spore-forming hyphæ from several contiguous mycelial branches, incline together, and twist themselves into a ball, as happens in the formation of a lichen thallus. These convoluted and contorted spore-forming hyphæ, being gelatinous, soon become so intertwined and entangled that they cease to be individually recognizable; to all appearances they coalesce together in part, if not entirely, and on the exterior of this gelatinous ball other hyphæ are now seen encircling it (Plate V. Figs. 10, 11). These latter, also being gelatinous, soon lose their individuality, although at times traces of their concentric arrangement can be made out. Sporeformation takes place only in the central gelatinous ball, in the middle of which it commences by the central part darkening in colour and becoming differentiated into spore-like bodies, which vary in number from four to sixteen. Apparently these bodies again subdivide, so * De Bary, "Vergleichung," p. 187.