63 



cortical cells ; it has the same color as the upper ends of the para- 

 physes. 



In the early period of the development of the apothecium this 

 layer, no doubt, plays an important part as a protective covering ; 

 whether it is of any use or not in the mature apothecia is not known ; it is 

 probable that the coloring substance has some influence upon sun- 

 light in its effects upon the maturation of the spores ; it may also 

 serve a protective function similar to the coloring substances deposited 

 in the thallus. 



(/?) The Thccinm. 



This layer occurs immediately below the epithecium and is the 

 most important tissue of the apothecium, because it contains the spore- 

 bearing thekes (spore-sacs, thecae). It consists of the paraphyses 

 and spore-sacs, both of which arise from the hyphae of the hypo- 

 thecium. The spore-sacs, according to some authorities, arise from 

 a specialized hyphal tissue known as the ascogenous hyphae. In 

 their early period of development it is possible to distinguish these 

 hyphae from the hyphae which produce the paraphyses ; they are 

 larger, more irregular in thickness, and the cell-walls are more 

 gelatinized ; tincture of iodine usually stains them blue, although 

 this reaction is by no means constant, and in the mature apothecium 

 this difference in structure and in chemical reaction cannot usually 

 be observed. The spore-sacs are the enlarged terminal cells of the 

 ascogenous hyphae which push their way between the paraphyses. 

 At maturity the sporogenous cell undergoes considerable chemical 

 change, especially in the upper and outer portion. In the Calici- 

 aceae the upper portion of the cell-wall is entirely gelatinized, even 

 before the spores have become mature. In the majority of cases 

 only the outer part of the membrane becomes gelatinized ; this 

 gelatinous portion readily absorbs moisture, causing the spore-sac to 

 enlarge and to aid in the expulsion of the spores. 



The mature spore-sacs vary greatly in the different lichen-groups ; 

 as a rule, they are somewhat shorter than the paraphyses and some- 

 what more expanded above than below ; in general outline they 

 may vary from long-cylindrical to nearly spherical. In position they 

 are placed vertical to the hypothecium and parallel to the paraphyses. 



The paraphyses are slender hyphal filaments usually colored at 

 the upper ends ; they are generally simple ; they may, however, be 



