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The Thallus, in its simplest form, consists merely of elongated 

 cells or filaments, Hvphae, among which the Gonidia are scattered, 

 and which lie under the surface of the bark on which the Apothecia 

 only are visible externally. Such a Thallus is called Hypophloeoid, 

 subcortical. It is sometimes made evident by a mere stain or dis- 

 coloration of the bark within which it grows. When the Thallus 

 becomes superficial (Ektophloeoid) it constitutes the horizontal 

 Thallus, and is scurfy, powdery, granular, crustaceous, often lobed 

 at the circumference, and scaly (squamulose.) The crustaceous 

 thallus is either uniform or cracked, (rimose and areolate,) or 

 mealy, (tartareous,) or warty, (verrucose,) or turgid. The hori- 

 zontal thallus is often surrounded by a pale or dark fringe or border, 

 the Hypothallus. The scaly thallus tends to become ascendant, 

 and to pass into the shrubby or fruticulose state, or into the still 

 depressed, foliaceous state, wliich constitutes its highest degree of 

 development. There are all degrees of transition of these different 

 states. All of them contain the essential features, the Hyphse and 

 the Gonidia, excepting a considerable number of parasitic Lichens, 

 in which the Thallus is wanting. . But it is with the higher forms 

 that the student can best begin his studies in the anatomy and physi- 

 ology of Lichens. 



For this purpose let him then take some foliaceous Lichen, a 

 Parmelia, for instance, and (being supposed to be acquainted with 

 the simple methods of manipulation, and to be possessed of a com- 

 pound microscope) let him make a thin section of the Thallus and 

 place it under a moderate magnifying power. He will see that it is 

 composed (looking from above) , (1) of a more or less thick and 

 dense cellular (parenchymatous) cortical layer ;" (2) of a stratum 

 of green cells (Gonidia) ; (3) of a filamentous tissue, more or less 

 compact, (Hyphae) in which the gonidia are imbedded ; (4) of a 

 parenchymatous inferior layer, to which are often attached fibrils, 

 by which the plant is attached to the substrate, and which in these 

 Lichens represent the hypothallus. All these parts are typically 

 present in all Lichens ; but sometimes the cortical parenchymatous 

 layer is deficient and the upper surface consists of a mere epidermis. 

 The inferior cortical layer is also sometimes absent, as in Peltigera. 

 The hyphoid or medullary layer is sometimes very compact, as in the 

 central cord of Usnea, but generally more or less cottony. In the 

 shrubby lichens the gonidia are on all sides of the stem, and the 



