Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/871

FUNGUS 835 surface of the tliallus, and the spermatia oil coming in con tact become united with it by a small tube through which the contents of the male organs fertilize the trichogyne. After fertilization the cells of the ascogonium increase in A FIG. " A, transverse section through the tliallus of C ollema microphyllum, with g, gonidia, and a, the point of trichogyne (x350); a,6, the point of trichogyne with spernmtium attached (x760). B, spores of Tfielidium minulu/um, the germinating filaments of which enclose the (?) hymetiial gonidia of Entiocarnon (After Stahl.) number and size, and the ascogenous hyphrc spring from the original coil and bear the characteristic asci in which the ascospores are formed. The relations between the fungal and the algal elements may be illustrated by the following instance from Dr Stahl s Beitrdge zur Entivickelungsgeschichte der Flechten. The Algae were formerly considered organs of the fungal portion, and received the name of gonidia. A smaller form of gonidia, called the hymenial-gonidia^ occurs in interstitial spaces of the apothecia of many Lichens; these hymenial-gonidia are the offspring of the ordinary gonidia (the thallus-gonidia), and have been carried up in the hymenium by the growth of its hyphae. When the asco spores are emitted from the apothecia, the hymenial-gonidia are cast out also, and falling in the neighbourhood of the ascospores are, many of them, enveloped by the germinating filaments proceeding from the spores, when the conditions are favourable for growth. Along with the growth of the hyphae, these hymenial-gonidia increase in size, and ultim ately act as the thallus-gonidia of the new Lichen. This process has been observed in Dermatocarpon Schcereri and Polyblastia rugulosa. When this discovery was made by Dr Stahl, he found growing beside the Dermatocarpon a species of Thelidium, the gonidia of which appeared to belong to the same species of Alga as those of the Dermato carpon. He isolated, under favourable conditions for growth, the spores of Thelidium and the hymenial-gonidia of Dermatocarpon, and soon obtained the thallus of Theli dium complete as regards gonidia and fructification. This experiment, by proving that the same species of Alga (in this case a species of Pleurococcus) served as the gonidia of two totally different fungi, also strongly supports the above views as to the dual nature of the elements composing the Lichen. These views, though supported by the most eminent botanists and backed by incontestable facts, are still occasionally the subject of attack by lichenologists. The suborder Lichenes contains a greater number of species than any other suborder or order of fungi, but it is probable that excessive subdivision has given rise to many. Lichens grow chiefly in exposed situations, such as the bark of trees and rocks, and are very generally distributed throughout the globe. They vary much iu size and form, but are generally of a f oliaceous or encrusting habit. They form a large proportion of the vegetation of alpine and polar regions. Their economic and medicinal properties are in many cases of importance. Order V. MYXOMYCETES. In the first stage of their life-history the Myxomycetes are mobile organisms, differing so strongly from any state of any other vegetable that it was proposed by Professor De Bary to place them among amoeboid animal organisms. In this stage, where one would expect a thallus of hyphae, a mobile plasmodium is found, which in habit of life greatly resembles animal organisms. In appearance it is slimy or creamy, and con sists of numerous anastomosing net-like channels, through which there is conducted with more or less rapidity a cur rent of protoplasmic matter containing many foreign bodies, such as particles of colouring matter of different natures, starch granules, diatoms, spores of fungi, &c. These FIG. 8. A, plasmodium of Didymium Ifucupiis. (After Cienkowski. X350.) B, closed sporangium of Arcyria incarnata. (After De Bary. x 20.) channels are not bounded by any definite membrane, and the direction is frequently changed, probably for the pur pose of gathering nourishment. Where one or more indi viduals are situated near each other these plasniodia occa sionally unite (this has been observed between two different species). These mobile masses ultimately, usually after undergoing division, are transformed into motionless fruits, in some cases of regular form (sporangia), and in others of irregular (plasmodiocarp). Compound fruits called athalia, of regular or irregular form, either naked or enclosed by a membrane (cortex), are produced externally on definite sporophores by cell-division, or within the fruit by free-cell formation. On germinating, the contents of the spores escape either in the form of a zoospore with a nucleus, vacuole, and long cilia, or of an amoeboid, and these zoo- spores or amocboids soon aggregate and form mobile plas- modia. Taking the two generations in their life-history, it will be found that the fructification has probably greater claims to a fungal nature than the plasmodium has to an animal. They have, therefore, been retained in the vegetable king-