REPRODUCTIVE ORGANS 159 



is partially or entirely immersed in the thallus or in the substratum on 

 which the lichen grows, and is either a globose or conical body wholly 

 surrounded by a hyphal wall, when it is de- 

 scribed as "entire" (Fig. 90), or it is somewhat _ 

 hemispherical in form and the outer wall is 

 developed only on the upper exposed part: 

 a type of perithecium usually designated by 

 the term "dimidiate." As the perithecial wall 

 gives sufficient protection to the asci, the 

 paraphyses are of less importance and are 

 frequently very sparingly produced, or they 

 may even be dissolved and used up at an early — ^^^^-^ ==5ieL^ — 



stage. The thallus of the Pyrenocarpeae is ^ 



often extremelyreduced,and the perithecia are F'g-90; A. entire perithecium of 



Pofinaohz'acea A. L.Sm.xca..^o; 

 then the only visible portion of the lichen. B, dimidiate perithecium of Aa-o- 



A few lichens among Graphidineae and co>-dm gemma^a Koerh. x c^. 20. 

 Pyrenocarpeae grow in a united body generally looked on as a stroma; 

 but Wainio^ has demonstrated that as the fruiting bodies give rise to this 

 structure by agglomeration — by the cohesion of their margins — it can only 

 be regarded as a pseudostroma. Two British genera of Pyrenolichens, 

 Mycoporum and Mycoporellum, exhibit this pseudo-stromatoid formation. 



C. Development of Reproductive Organs 



As most known lichens belong to the Ascolichens, the study of develop- 

 ment has been concentrated on that group. Tulasne^ was the first to make 

 a microscopic study of lichen tissues and he described in considerable detail 

 the general anatomical structure of apothecia and perithecia. Later, Foisting^ 

 traced the development of a number of perithecia through their different 

 stages of growth, but his most interesting discovery was made in Lecidea 

 fumosa, a crustaceous Discolichen with an areolate thallus in which the 

 apothecia are seated on the fungal hyphae between the areolae. In the very 

 early stages represented by a complex of slender hyphae, he observed an 

 unbranched septate filament with short cuboid cells, richer in contents than 

 the surrounding filaments and somewhat similar to the structure known to 

 mycologists as "Woronin's hypha," which is an ascogonial structure. These 

 specialized cells disappeared as the hymenium began to form. 



1 Wainio 1890. ^ Tulasne 1852. " Fuisting 1868. 



