66 



POPULAR HISTORY OF LICHENS. 



The pedicle at its free end is frequently bent or twisted ; 

 this is a remnant of the original development of the theca, 

 which springs laterally from a hypothecial cell. The form 

 of the theca depends chiefly on the state of maturity of its 

 contents ; in the young state, it is generally more or less 

 narrow, but it becomes regularly or irregularly dilated as 

 the spores are developed. The effect of maturescence of the 

 spores, in producing a marked change of form in the theca, 

 may be easily studied in Parmelia jparietina or Physcia 

 ciliaris. The thecal wall is somewhat thick ; its thick- 

 ness is greatest in the young state, and at the apex of the 

 theca; hence iodine, which tinges it, on account of its 

 starchy character, of a beautiful prussian-blue colour, de- 

 velopes the greatest intensity of tint at the apex, or superior 

 extremity of comparatively young thecae. The inner surface 

 of the thecal wall is normally lined by a delicate membrane, 

 differing from it in its chemical characters, which forms 

 a shut sac or cell, similar in size and appearance to the 

 theca itself, and which is called the spore-sac. This mem- 

 brane is not only closely applied to the interior of the thecal 

 wall, but is frequently incorporated with it ; it can seldom 

 be recognized as a distinct membrane, unless under the 

 action of chemical reagents. It would appear — probably 



