V1U 



they are attached, supplies to them in the state of very complex 

 solutions the substances necessary to their existence. Their 

 growth is, therefore, necesssarily slow and intermittent, depending 

 on the dryness or humidity of the atmosphere, on dry or rainy 

 seasons. In the earliest stage of their existence their growth is 

 very rapid under favourable conditions, but when they have at- 

 tained to a certain size, growth would seem to be arrested and 

 becomes slow. Their duration of life, especially in the fecundity 

 of the reproductive organs, is remarkable and excessive, probably 

 extending to many centuries. They will not grow in a flourishing 

 or perfect condition in the immediate neighbourhood of towns, or 

 where the air is impregnated with smoke, soot, or other deleterious 

 ingredients. In such situations they exist only in a gonidial or 

 rudimentary state, appearing on the trees, walls, &c, as green 

 dust, in which state they will continue for ages, increasing by 

 bisection like the Algas, but never developing into perfect lichens. 

 The pseudo-genus Lepraria of the elder botanists represented this 

 condition. Their abundance in a fully developed and fructiferous 

 condition is a sure and certain indication of the purity of the air 

 and salubrity of the climate. 



A lichen consists of a thallus or vegetative portion, bearing the 

 fructification in the apothecium or female fruit, and the spermo- 

 gonium or supposed male fruit. There are also other bodies, 

 occasionally found on the thallus, termed pycnides, which some 

 consider as a kind of secondary fruit, and others as fungilli. 



The thallus is very variable in form, size, colour, and consistence, 

 and is either horizontal and membranaceous or foliaceous, or 

 pendulous and filamentose, or occurring in small erect shrublike 

 clusters, or else forming a more or less thick crumbly or powdery 

 crust, investing the surf ace of rocks and trees, and becoming lobed 

 in various modes in the circumference. If, then, we make a vertical 

 section through a membranaceous thallus, it will be seen to con- 

 sist generally of a cortical layer on the upper surface formed of 

 minute, closely compacted cellules, immediately underneath which 

 is the gonidial layer consisting of bright-green spherical cells 

 arranged in an interrupted manner, immediately above the 

 medullary layer which consists of interlacing tubular and articulate 

 colourless filaments. The lower surface is either naked, or has a 

 cortical layer similar to the upper one, and also rhizince or 

 rootlike filaments which attach it to the surface upon which it 

 grows. 



If again we make a similar vertical section through a crustaceous 

 thallus, it will exhibit a cortical layer on the upper surface with a 

 gonidial layer immediately underneath,!and then the medullary layer 

 which instead of consisting of filaments has become dead, and 

 transformed into a tartareous mass of abundant molecular granula- 

 tions, often of oxalate of lime. Underneath the entire lichen is 

 the hypothallus, generally of a black or dark-brown colour, formed 

 of filaments or compacted cells, and which in fact is the primordial 



