284 LETTER XXV. 



ceedingly minute cells that arc lodged in the centre of 

 the shields. These are very difficult to find ; you 

 may, however, make them out if you observe the 

 following directions. Take the full groTvn shield of 

 any Lichen ; that of the Yellow wall Parmelia (P. 

 parietina) is a good one for the purpose (Plate XXV. 

 1. Jig. 2.) ; with a sharp razor divide it perpendicu- 

 larly ; then shave off the thinnest possible slice of one 

 of the faces, and drop it into water ; place it on the 

 glass stage of a microscope, and illuminate it from 

 below. You will then be able to perceive that the 

 kernel consists of a crowd of minute compact fibres, 

 planted perpendicularly upon a bed of cellular sub- 

 stance {jig. 3.) ; and that in the midst of the fibres 

 there is a great number of little oblong bags {Jig. 3. 

 a.) filled full of transparent cells {Jig. 4.) ; the bags 

 are thecse, the cells are spores ; and it is to the latter 

 that the Lichen has to trust for its perpetuation. 



The study of Lichens is probably the most difficult 

 of any part of Botany ; the species are scarcely to be 

 distinguished, the limits of the genera are uncertain, 

 and the characters by which they are separated are 

 obscure. If, however, you are curious to make 

 yourself acquainted with these, the best book I know 

 of to recommend to you, is the fifth volume of the 

 English Flora, by Professor Hooker. 



Notwithstanding- their minuteness and uninvitino- 

 appearance, several of them are of considerable im- 

 portance to man and animals. The Arctic Gyro- 

 phoras called by the Canadians Tripe de Roche., were 

 the only food that the daring travellors Franklin, 



