J 95 



in Leftogium both the upper and lower cortical tissue is developed, 

 though it is usually one cell-layer in thickness; it is very charac- 

 teristic, closely resembling in appearance the epidermal layer of a 

 leaf ; in certain areas it consists of more than one cell-layer. As in 

 Collema the algae are more numerous near the upper surface of the 

 thallus. 



The structural differences are more marked in the apothecium ; 

 the cortical layer of the thalloid exciple is well developed ; the hypo- 

 thecium is quite thick and cortical in structure. 



The spores have the same general characters as in Collema ; they 

 differ in that they are usually more pointed at the ends ; they lie 

 parallel to each other diagonally across the spore-sac, which is also 

 true of Collema spores as far as I have been able to observe. 



The algal characters are the same as in Collema. The habitat 

 of the species and their distribution is the same as in that genus. 



PLATE 62. 

 Leptogium tremelloides Fr. 

 i . Thallus natural size. 



2. Portion of thallus-lobe with apothecia, magnified. 



3. Section of apothecium. 



4. Section of thallus. 



5. Paraphysis and spore-sac. 

 6- Spores. 



7. Algae. 



3. Mallotium Ach. ; S. F. Gray, Brit. PI. 1:399. 1821. 

 Vegetatively this group represents highly evolved forms ; it 

 presents a much higher development than Leptogium, as is indi- 

 cated by an increase in the size of the thallus ; the cortical layers are 

 also more developed. The algae (Nostoc) indicate a marked ten- 

 dency to accumulate near the upper surface of the thallus. The es- 

 sential character which distinguishes this genus from Leptogium is 

 the presence of numerous long, comparatively rigid, gray rhizoids 

 which extend in clusters from the lower surface of the thallus ; they 

 partly serve as organs of adhesion and as special organs for absorb- 

 ing moisture from the substratum and from the air; incidentally 

 they may also serve as organs of protection by keeping off crawling 

 insects. The bitter lichenic acid which occurs abundantly in these 



