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). 

 * Lichen Fhra of Great Britain, p. xvii. (3rd ed. 1879). 

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