Page:The American Cyclopædia (1879) Volume X.djvu/415

 LICHENS 409 presents first a layer of cells of colorless cel- lular tissue, the cortical layer (c T). Beneath this is the gonidial layer, made up of opaque cells, not altogether continuous, called gonidia (g) ; these are usually bright green or olive- green, and their presence is characteristic of lichens, serving to distinguish them from fungi. Next below the gonidia is the medullary lay- er (m /), consisting of elongated cells, which are either (1) felted or interlaced to form a loose web, (2) crustaceous, when the filaments are fewer, accompanied by white granules and crystals of oxalate of lime, or (3) cellular, when rounded or angular utricles are associated with the filaments. The lower layer of the thallus, called the Jiypothallus (h is of cells or filaments, and is usually darker than the upper surface ; it sometimes gives off root- like hairs (rhizines) which serve to attach the lichen to its matrix. Various modifications of this structure are met with ; in some the cor- tical and in others the lower layer is wanting ; in the erect lichens the medullary layer serves as an axis around which the gonidia are ar- ranged, and in some of the crustaceous lichens it is difficult to distinguish any other elements than the goni- dia. The or- gans of fructi- fication, called apothecia, are sometimes con- cealed within the tissues of the thallus, but are commonly upon its sur- face or margin, where they ap- pear as vari- ously shaped disks, to the different forms of which the descriptive names of peltate, scutel- late, &c., are given ; sometimes, as in what are called the " written " lichens, gr aphis (fig. 4), for example, the apothecia are elongated or branch- ing irregular spots, which have been compared to Japanese letters ; similar lichens of related genera are quite common on the bark of oak and other forest trees. Apothecia are rarely of the same color as the thallus, and are black, brown, yellow, or red of various shades. A general idea of the structure of an apothecium may be had from fig. 5, showing a magnified section. The microscope shows a number of oblong or club-shaped bodies, the spore cases (* c) or asci, to which all other parts of the apothecium are subordinate ; these spore cases (also called thecce and sporangia) are surrounded by numerous filaments, the parapTiyses (par} ; the exterior of the apothecium has a cortical layer (c l below which are gonidia (g as in the thallus. Other apothecia vary in form and details from the one figured, but their office is the same, the protection and development of the spore cases, which contain the spores. Though lichens multiply by other methods, FIG. 4. Graphis elegans. that by spores is regarded as the normal one, corresponding to the reproduction of flowering plants by seeds, while the other ways in which they are multiplied are equivalent to propaga- ting by means of cuttings, &c. The common FIG. 5. Microscopic View of the Cross Section of an Apo- thecium. s c, spore cases; par, the filaments or para- physes accompanying them ; c I, cortical layer ; g, gonidia. number of spores in each spore case is eight, but there are in some species six, four, or two, and even a solitary spore ; on the other hand, they are (rarely) more numerous, up to 100 or more. The spores vary greatly in size, being inch in length ; in form they vary from globose to needle-shaped, and in color from brownish yellow to nearly black. Some spore cases with their paraphyses are shown in fig. 6. One or more subdivisions are to be seen in the spores themselves, characters that are made use of in describing these plants ; the color of the spores is very constant, and furnishes good distin- guishing marks for gen- era and species. When the spores are perfect- ed they are thrown out from the apothecium, FIG. 6. Spore Cases with paraphyses, highly mag- nified. FIG. 7. Microscopic View of the Cross Section of a Pycni' dium. either with or without the enclosing spore cases. Many lichens bear upon the thallus