274 LETTER XXIII. 



plants, or races very closely allied to them, grew in 

 ancient days, long before the creation of man, to a size 

 far beyond anything that the present order of things 

 comprehends ; this is, however, a geological matter, 

 with which we had better not interfere. 



EXPLANATION OF PLATE XXIIL 



L The Fern Tribe. — l.Theupperendof aleaf ofZTar^s-I'oM^we 

 (Scolopendrium officinarum), seenfrora the under side; csori. — 2. A 

 portion of a leaf magnified, to show the veins and the structure of the 

 sori, a a, more distinctly. — 3. A similar portion, further advanced; 

 a a sori. — 4. A section of a portion of a leaf across the sorus; a a 

 the two valves of the indusium; h the sorus itself, covered with 

 thecae. — 5. A theca; a its elastic back. — 6. Another theca burst, and 

 scattering its sporules, a. 



II. The Club-moss Tribe. — A plant of common Cbib-moss (Ly- 

 copodium clavatum). — 1. A theca, with its two valves open, and the 

 leaf out of whose bosom it grows. — 2. The same leaf seen from be- 

 hind. — 3. A theca, as seen when closed. 



