CHAPTER II.—DIFFERENTIATION OF THE THALLUS.—MYCELIAL LAYERS. 21 
mucor-tubes which they encounter, and enter into open communication with them at 
this point by the dissolution of the cell-membranes and complete coalescence of the 
protoplasm of both plants. At these points of union they now put out small vesicular 
projections, which in strong specimens appear in numbers close together and form clusters 
which may reach the size of a pin’s head. It is obvious that these vesicles do not, like 
the haustoria in the previous cases, serve as organs of attachment and nutrition, for 
organs of the kind are rendered unnecessary by the-union of the parasite and the host. 
They are evidently storehouses of food-material, and the fertile branches of the thallus 
spring chiefly from them. But in relation to the morphological points at present under 
consideration they are in their nature essentially branches of the mycelium, which 
however stand in the closest and most exclusive relation to the physiological function 
above mentioned. 
Organs of attachment of an unusual kind resembling haustoria are peculiar to the 
species of Sclerotinia which have been examined, S. tuberosa, S. Sclerotiorum, 
S. ciborioides, S. Fuckeliana, and also to the gonidial state of this species known 
as Botrytis cinerea. Under conditions to be described in the sequel the mycelium 
of these plants, often when still quite young, forms short branches on which arise 
tufts of secondary branches, which becoming closely clustered together are divided 
“ by numerous transverse walls into short segments with membranes that become dark 
brown with time. The clusters may be of the size of a pin’s head, and have then 
been mistaken for sclerotia, with which however they have no connection. They 
are formed when the mycelium under conditions of plenteous nourishment is‘ growing 
on a solid substratum, such as a plate of glass, which it cannot penetrate, and they 
apply themselves closely to the substratum. On substances into which the plant 
penetrates, such as the parts of plants which are suited to it, the tufts are not formed 
at all or are only feebly developed, in which case their branches soon pass into the 
substance of the host and grow there into slender branches of the mycelium. Brefeld 
gives figures of these formations in his Schimmelpilze'. 
Section VI. The mycelial hyphae of many Fungi, when the conditions are 
favourable, become interwoven with one another and form membranous layers 
which may be of considerable extent and thickness. 
This is the case with such Hyphomycetes as Aspergillus niger, A. clavatus, and 
Penicillium glaucum, which in their simpler condition have a filamentous and floccose 
mycelium, if they grow on the surface of a moist nutritive substratum. They some- 
times form large expansions on the surface of fluids, and may be lifted off them like 
a cloth. The free surface of the mycelium is in these cases usually clothed with the 
filiform sporophores. 
A second series of examples is supplied by many, perhaps by the larger part, of 
the solid and especially of the woody and wood-inhabiting Hymenomycetes, the 
mycelia of which form very thick membranes or crusts, sometimes of considerable 
breadth and some millimetres in thickness, on the free surface of the substratum or in 
clefts inside carious stems of trees. Sporophores spring on the one side directly 
from the membranes, and on the other single filaments or bundles of filaments branch 
off from them and penetrate into the substratum. Other instances occur here and 
there in other groups, and are mentioned in special publications?. 
Apart from the exceptional case of Agaricus melleus which will be described 
below, the only general remark of importance upon the structure of these mycelial 

1 Schimmelpilze, IV, t. IX. 
* See the literature cited at the end of the chapter. 
