fv. ] Lie HEXES. 359 



6. Natural Order, LicJienes. — The Lichen Family. 



Lichens occur either as crust-like or leafy expansions, 

 Dr in little branching shrubby tufts, usually coloured grey, 

 yellow, or greenish-yellow. They spread ever}-\vhere in cool 

 climates — over stones, brick-walls, the bark of trees, and 

 even upon the most exposed rocks of alpine and arctic 

 countries, forming the verj' outposts of vegetation, and 

 growing at the expense, almost solely, of the atmosphere 

 and the moisture which it bears to them. In the tropics 

 the relative proportion of lichens growing upon the leaves 

 of trees {epiphyllous lichens) is large, and gives a special 

 character to tropical Lichenolog}'. Lichens are long-lived, 

 and intermittent in their growth,, being at a standstill, and 

 often crumbling away, when the weather is dry. They differ, 

 also, from Fungi in containing a green-coloured layer under 

 the epidermis, consisting of cells called gonidia, which may 

 be regarded as answering to the buds of higher plants, since, 

 when set free, they develop new lichens, and thus multiply 

 the plant. The true reproductive organs are contained in 

 special receptacles, either exposed upon the upper surface of 

 the lichen or buried in its tissue, the spores being contained 

 in narrow cells similar, in some species, to the asci of 

 Sporidiferous FungL 



A transverse section through the crust {thallus) of a 

 typical Lichen exhibits under a magnifying power of 400 

 or 500 diameters immediately underneath the upper cortical 

 layer of rather thick-walled cells, a stratum largely made 

 up of green chlorophyll containing rounded cells, either 

 scattered singly or, more usually, clustered or joined end to 

 end ; these, under the name of '* gonidia " have been hitherto 

 regarded as affording a means of vegetative propagation 



