944 



XII. LICHENES. 



the middle layer of 

 the thallus {hypo- 

 thecium). The di- 

 mensions of the 

 sporangia vary 

 much according to 

 age, to genera and 

 species, and to the 

 number of spores 

 which they con- 

 tain. Their wall 

 is formed of a 

 rather thick mem- 

 brane, especially 

 when young, of an 

 extreme tenuity in 

 certain genera; in 

 others it atta.ins 

 •00039 in., and 

 their greatest 



thickness is usually 

 top of the sporangium. 

 They persist after emitting the 

 spores. 



There are usually 8, less com- 

 monly 6-4-2 spores in each sporan- 

 gium, but sometimes 20-100 spores, 

 and even more, occur; as does a 

 solitary spore. The spores vary in 

 size according to their number. 

 When 8, they measure from '00027 

 to-00167, by -00007 to -OOOOG of an 

 inch. The smallest known are not 

 more than '00003 in. in length, 

 and half as much in breadth. The 

 spores are ellipsoid, ovoid, fusiform 

 or oblong-cylindrical ; they are 

 simple, or chambered and 2-8-4- 

 co-celled. Two layers may be 

 distinguished in their walls^^an outer or efispore, aijd an inner or endo- 

 spore. The epispore is usually extremely thin, and scarcely perceptible'. The 

 colour of the spores is always determined by that of the epispore ; white is most 

 usual. Iodine often colours them blue ; but in all cases the epispore alone becomes 

 coloured. The spores are expelled by a peculiar contraction of the sporangium, in 



Roccella Hndoi'ia, 



