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 she 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 kmd 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 2 . 



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. 



2 See the literature cited at the end of the chapti i 



