18 DIVISION I.—GENERAL MORPHOLOGY. 
2. THE MYCELIUM. 
Section V. The mycelia in their original form are always free hyphae; they 
either retain this character during their whole life, or the hyphae as they grow 
become at most loosely interwoven with one another without forming bodies with a 
definite shape and outline, filamentous or floccose mycelia; or the hyphae form by 
their union elongated branching strands (fibrous or fibrillose mycelia), or membranous 
expansions, or tuber-like bodies, sclerotza. 
The filamentous mycelia are much the most common, and in the majority of 
Fungi they are the only known form. Their character has been already described in 
speaking in the first chapter of the hyphae of the Fungi generally. The branching of 
the mycelial filaments in all cases that have been observed with certainty is mono- 
podial. The phenomena of coalescence of hyphal cells that were originally free, and 
of clamp-connections which were described above, appear as a rule in their most 
striking form in filamentous mycelia. 
Differences in the structure of mycelial filaments must necessarily depend 
chiefly on the presence or absence of a regular system of transverse walls, and this, as 
has been already intimated, varies in the different groups. (See also Chapter V.) Every 
species in each of the two chief categories thus obtained exhibits as a rule its own 
peculiar phenomena of growth and differentiation, provided the normal conditions of 
growth remain unchanged, and by these phenomena the several species and groups of 
species can be distinguished from one another. These differences relate to the 
average size and special form of the cells, the divergence of the branches, the phenomena 
of coalescence and the like. Owing to the diminutive size of the objects, they are 
usually very inconspicuous even under the most favourable conditions of growth, 
and to ascertain them with certainty requires careful observation. They are liable also 
to so many changes from external causes that the determination of a mycelium, which 
under favourable conditions of development has well-marked characters, without its 
sporophore may be attended with considerable difficulty in practice, if it has to be 
observed under less favourable circumstances. Much advance has been made in this 
point of late years through the careful examination of individual species, so that we 
may expect that the morphological characters of the mycelia of many species 
and groups of species will in time be clearly determined. 
Some filamentous mycelia, belonging to species from very distinct groups, are 
distinguished by having special organs of attachment and suction, known as haustoria; 
these are peculiar branches which attach the mycelium firmly to the substratum, 
and in most cases also evidently serve to take up nutriment from it. Such organs are 
found in many, but by no means in all, parasitic species living on plants and 
belonging to very different groups, as the Peronosporeae, Piptocephalis, the 
Uredineae, and Erysipheae. 
The mycelial filaments of these Fungi spread themselves on or among the cells 
of the host ; the haustoria are formed on them as special lateral branches which force 
“their way into the interior of the cells; they vary in form according to the species and 
are more or less, often extremely, unlike the extracellular hyphae. Organs of attach- 
ment, which at least very nearly resemble the haustoria of these parasites, are found 
in a few other non-parasitic mycelia; they will be noticed again subsequently. 
