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 BOTANY having a siliceous epidermis which is incombustible and not subject to decomposition. They are a most interesting group, partly on account of their movements, which differ altogether from the movements of desmids. THE LICHENS (Lichenes) Lichens are cellular plants intermediate between the algae and the fungi, having a thallus like that of the fungi, and gonidia by which they are allied to the algas. The vegetative structure developed from the reproductive cell is called a mycelium, and on the hypothesis that the filamentous hyphae of which it consists are parasitic upon algal gonidia the theory has been broached that the lichens are merely a class of para- sitic fungi, nearly allied to the Ascomycetes or even belonging to them. This 'dual-lichen hypothesis' of Schwendener is endorsed by Sachs, 1 but it is regarded by the highest authority on the British lichens, the Rev. W. A. Leighton, 2 as ' the baseless fabric of a vision.' The thallus of the lichens consists of three cellular layers : (i) the cortical layer on the upper surface, forming the outer covering of the thallus ; (2) the gonidial layer, consisting of bright-green spherical cells ; and (3) the medullary layer, the colourless cells of which enclose the gonidia on the under side of the thallus. This layer varies much in structure in different kinds of lichens, its lower surface sometimes having rootlike filaments which attach it to the surface upon which it grows but do not obtain nourishment from it as do the homologous filaments of the fungi. The gonidia vary in colour from a yellowish to a bluish green, the chlorophyll or other green granular matter which they con- tain being generated by the action of light. The reproductive system is too complicated to be described here, but it may be mentioned that the spores are contained in asci or thecae, as in the Ascomycetes, and usually are eight in number. Lichens derive all their nutrition from the atmosphere, and for their perfect growth require a pure air. They give beautiful patches of colour' to the trees and rocks, old palings and walls, stones and earth, on which they grow, most luxuriantly in damp situations. Our knowledge of the lichens of Hertfordshire is chiefly derived from a few records in the Transactions of the Hertfordshire Natural History Society of species found at field meetings of the society in Bricket Wood and the Tunnel Woods, Watford, and from a manuscript list by the Rev. W. H. Coleman of species found in the neighbourhood of Hertford. The noteworthy species are Calicium melanophceum found on fir trees in Bricket Wood, Peltigera polydactyla on moss-covered ground in Oxhey Wood, Lecanora phlogina on trees in the Tunnel Woods, and Pertusaria globulifera on trees in the same woods and also in Bricket Wood. The extensive woods of Wormley and Broxbourne in the east of the county 1 Text-book of Botany, p. 262 (1875). 69
 * Lichen Fhra of Great Britain, p. xvii. (3rd ed. 1879).