formed under normal circumstances; consequently there is, in this case, no separation of the elongating tubular bud into inner and crown cells. In such event the resulting mycelium consists, as already stated, of a single, many-branched, tubular, nonseptated cell, such as is shown in Fig. 91.

The foregoing statement that the inner cells do not play any further part in the mycelial growth of the Mycomycetes, inasmuch as they neither extend in length nor develop septa, may be taken as the rule. There are, however, exceptions, septation, accompanied by elongation, frequently occurring within the inner cells in the event of abnormal conditions of nutrition. This phenomenon is termed intercalary growth, or intercalary septation, to distinguish it from acrogenous growth.

If, in the absence of external causes of hindrance, the growth of the mycelium is able to proceed equally in all directions, a stellar system of radial, branched threads, with the spore as a centre, is the result. This form of growth was termed a typical mycelium by Zopf. The practical worker in a mycological laboratory can obtain such typical mycelia in a youthful condition, and consequently easy to survey, if he re-examines, after a lapse of one or two days, the plate cultures (§ 85) that have already been examined for the purposes of mycological analysis (e.g. of water, milk, beer). During the first investigation the spores of all kinds of mould fungi from the air will have settled on the solid nutrient medium, each of them then germinating to form a mycelium, and thus yielding, as it were, a self-prepared culture.

Mention must here be made of one of the various instances of irregular mycelial development, since it will have to be referred to on a subsequent occasion: this is the phenomenon of intergrowth. It is caused by one of the cells in a mycelium putting forth a branch into the interior of an adjoining cell, so as to displace the intervening septum. The invader may then become divided into cells within the plasma of the invested cell, with the result that an inexpert observer may easily be led to believe that endogenous spores are present. An example of this growth is represented in Fig. 94. Another will be found in a later section dealing with Dematium pullulans, and a third in the case of Oidium Ludwigii Hansen, occurring in mucilaginous discharges from trees (§ 248), and investigated by W. HOLTZ (I.). This was probably also the method of formation of the alleged spores observed by EDM. KAYSER (V.) within the hyphæ of an unknown mycelial fungus isolated by him from fermenting pineapple juice. Intergrowths also occur in the sporangia of several fungi.

In the case of a large number of fungi, the development of the mycelium ceases with the formation of the branched hyphæ, the ensuing process being the elaboration of organs of fructifica

tion. Fungi exhibiting this class of simple mycelial structure are classed under the generic name Hyphomycetes or Thread Fungi. The term Mucedinæ, occurring in the French and English literature, expresses about the same thing. It may be remarked in passing that several botanists, e.g. Strasburger, Noll, Schenck, and Schimper, in their botanical text-book, employ the name Hyphomycetes in a far wider sense, namely, to include the whole of the Eumycetes, the reason for this being that the production of hyphæ is characteristic of these fungi, and constitutes a fundamental distinction between them and the other divisions of the fungus family, the Schizomycetes in particular. Nevertheless, in the following pages we will apply the term in its more restricted In many of the other classes of Eumycetes, the development of the mycelium does not cease at the stage we have described as the typical mycelium, but extends further, to the production of

sense.

FIG. 94.-Botrytis cinerea.

Intergrowth. Each of the two penultimate cells of the depicted fragment of mycelium has grown into its neighbour, and there become separated into globular cells. The central cell of the mycelial thread has put forth abnormally developed organs of fructification. (After P. Lindner.)

66

aggregations, of the forms known as mycelial threads and mycelial films. A combination of these two forms constitutes the large bodies known in colloquial language as 66 mushrooms or fungus"; the botanist, however, terming them fungoid bodies. The capacity of forming such bodies, upon or within which the organs of fructification are situated, is confined to the most highly developed species of fungi. An example is given in Fig. 95. This, however, is only one variety (though appearing in numerous modifications) of the coalescence and intertwined growth of hyphæ, another form being that of the so-called sclerotium or hard mycelium. The well-known ergot of rye, which will be more closely described in the last section but one, forms an example of a sclerotium. This is constructed of closely intertwined hyphæ, furnished with a store of nutrient material, and constituting a hard permanent form, which, after a variable period of quiescence, awakens to active life, puts forth organs of

fructification, and is then able to await and utilise the occurrence of favourable conditions, in order to effect the reproduction of the individual from which it has originated. An observation on the artificial production of such permanent forms has been communicated by J. RAY (I.). Among the foodstuffs accumulated in the

cells of the sclerotium, special importance attaches to glycogen (§ 253) as the source of easily liberated chemical energy and abundant disengagement of heat. This substance was first observed-without, however, being specially named-by A. DE BARY (1.) in the sclerotium of Coprinus stercorarius; and it was afterwards found, by W. ROTHERT (I.), in that of Sclerotium hydrophilum. A thin section of sclerotium, or of a fungoid body-both of which are, as already stated, composed of a network of hyphæ— exhibits under the microscope an appearance similar to that of the parenchyma of higher plants, e.g. a section through the flesh of an apple. On account of this similarity, these networks of hyphæ have received the name pseudoparenchyma, which, however, is not intended to express any further likeness, whether in respect of the mode of formation or physiological purpose. It is perhaps superfluous to emphasise that the mycelia of this class of fungi consist merely of hyphae when in the earliest stages of existence and consequently at such times are indistinguishable in this respect from the mycelia of the Hyphomycetes.

FIG. 95.-Boletus edulis.

Longitudinal section (above) and transverse section (below) through the fruit stem ("fungus") of Boletus edulis. Magn. 00. (After Strasburger.)

§ 219.-The Gemmating Mycelium.

The application of the name "typical" to a mycelium growing in the manner described in the preceding paragraph, indicates the possibility of other methods of growth, manifesting themselves as modifications and simplifications of this form. Of these the most important, from our point of view, is the gemmating mycelium, the development of which proceeds in the following manner (Fig. 96): The germ cell, or mother-cell, puts forth a protrusion which, however, instead of enlarging to a tube

as in the case of the incipient typical mycelium, assumes a form resembling that of the parent-cell and therefore termed a bud. The daughter-cell then becomes divided from the parent by a septum, which subsequently splits into two layers and enables the two cells to separate. In many instances the parent-cell puts forth only a single bud, but in others two or more. As soon as the daughter-cell has attained the size of the parent, it is then able to behave in turn like the latter, and itself put forth a bud (of the second order), from which again proceeds another bud (of the third order) and so on.

a

If the parent-cell-as is the case, for example, in most kinds of yeast-be globular, oval, or lemon-shaped, the daughter - cell will also usually be of similar form, and is then termed a short bud. Such globular buds. are referred to in the older literature (and occasionally even now) as spherical yeast, more particularly in the case of Mucor. If, on the other hand, the parentcell be of elongated form, the daughter-cells issuing therefrom will preferentially develop in a longitudinal direction from the start, and thus form elongated buds. Most of the species of Mycoderma afford examples of this type. Fungi with gemmating mycelia of this kind are therefore, in this respect, intermediate to the fungi with typical mycelia.

FIG. 96.-Gemmation of a Tortula in Beer wort.

At (a) one of the cells has just put forth a tiny bud. At the end of hours (b) this has two hours it has grown to half the size of the become considerably larger. After another

parent cell, and has already separated from the latter. Magn. 1000. (After Hansen.)

The above-mentioned double stratification of the septa between the cells produced in the foregoing manner, permits these cells to enjoy an independent existence, and consequently enables them to be separated from one another. In many instances this actually occurs, and consequently the nutrient medium wherein this takes place, will exhibit a comparatively large number of single cells. Conversely, in other instances, the successively developed buds remain connected together, forming a cellular aggregation (Fig. 97). In the older literature, such aggregations, when composed of globular cells, and therefore resembling a series of small knots (Lat. Torula), were generally named Torulæ. This was afterwards employed as the generic name for a number of species, some of which are capable of exciting alcoholic fermentation and will be described in a later section. An example of these is given in Fig. 98.

The form of the gemmæ from one and the same species is also dependent on the temperature and the conditions of nutrition, as has been shown by E. Ch. Hansen in the case of beer yeasts and wine yeasts. These, when submerged in beer wort, develop

mycelia constructed of short gemmæ; whereas, when cultivated on the surface of the liquid, and therefore in presence of abundance of air, they form mycelia composed of elongated buds. Further particulars of this will be found in § 246.

The formation of mycelia composed of gemmæ was first observed in the case of yeast fungi, and was regarded as a method of development peculiar to these organisms. BAIL (I.), however, in 1857, showed that this phenomenon also appears in certain species of Mucor (see Chapter xliv.) when submerged in

FIG. 97.-Saccharomyces pyriformis Ward.

Cell a, embedded in a hanging drop of gingerbeer gelatin and kept at a temperature of 15° C., threw out a bud (8) within 4 hours. At the end of another 14 hours three normal cells (y) were present, which grew to the aggregation & in another 10 hours. This in turn had developed into the colony e in 13 hours more. (After M.Ward.)

a nutrient solution containing sugar. For more precise observations of this phenomenon in the case of Mucors, we are indebted to BREFELD (XVI.). With Mucor racemosus, the carbon dioxide collecting in the nutrient fluid acts upon the cells, by which it has been produced, in such a manner that the latter put forth none but spherical gemmæ, and therefore grow, not to a long and many-branched, unicellular mycelium, but to one composed of stumpy gemmæ. On the other hand, Mucor mucedo treated in the same manner does not produce similar gemmæ, though, according to BREFELD (XVI.), its spores, when placed in a nutrient solution rich in citric acid, swell up to large globules from whence proceed a number of similarly