28 



places where books are not accessible, and yet where this 

 little essay may easily find its way. No other apology, there- 

 fore, will be needed for reprinting Dr. Hooker's remarks. 



Lichens are a very large natural order, found in all 

 climates and latitudes. "Upwards of 1,300 species are de- 

 scribed, very many of which have most extensive ranges 

 of distribution, from the Arctic Circle to the Equator, in 

 both hemispheres. Like most of the Cryptogamic orders, 

 they chiefly affect damp temperate climates. They are com- 

 posed of a thallus, which usually spreads horizontally over 

 dead wood, bark of trees, or rocks. In many genera the 

 thallus is erect or pendulous, and sometimes it grows from 

 the earth like moss. The thallus may be (1) effuse, that is 

 without determinate shaj^e ; or (2) effigurate, determinate in 

 shape ; (3) scaly, formed of small coriaceous scales ; (4) 

 crustaceous, of a thick crusty substance ; (5) poivdery, or lej)- 

 rous, when formed of minute membranous scales ; (6) granu- 

 lar, composed of minute granules ; (7) foliaceoiis, forming 

 leaves like branches. The attachment of the thallus may be 

 by the whole under surface, or by fibrils or bundles of short 

 filaments, and may be by one point or many. Wiien the 

 thallus is erect it is often called a podetium, a name given to 

 the erect cylindrical portion of a horizontal thallus. 



Four kinds of reproductive organs have been noticed in 

 Lichens, viz. : — 1. Aj^othecia, circular or variously shaped 

 shields, cups, or prominently formed of closely packed jointed 

 filaments and closed tubes (asci), which contain simple or 

 septate spores. 2. S^ermagoyies, minute open cavities in the 

 thallus, containing filaments (sterigmataj, upon which are 

 extremely minute colorless bodies called siiermatla. 3. Fyc- 

 nidia, or superficial sijermagones. 4. Goniclia, or granules, often 

 scattered like powder over the thallus ; they are analogous to 

 buds, and reproduce the species. 



The internal substance of the thallus consists usually of 

 three layers : — 1- The corticle, which is tough and leather- 

 like, formed of densely packed minute cells, with thick walls. 

 2. A green gonidia layer, formed of loose bright green or 

 yellow globular cells, which either have a proper cellular 

 coat, and are called gonidia, or have none, and are called 

 gonina. These cells are almost peculiar to Lichens. They 

 often burst through the upper layer in masses called soredia, 

 or are scattered like i:>owder over the cortical layer, or fringe 

 the lobes of the thallus. In the genus Sticta they burst 

 through the under surface of the thallus, and occupy small 

 circular depressions or cups called cyphellse. Many Lichens 

 are extensively propagated by gonidia, which may be seen 

 forming green or yellow powdery strata on bark, stones, etc. 



