STRUCTURES PECULIAR TO LICHENS 



129 



There seems no doubt that the chief function of these various structures 

 is, as Schwendener 1 suggested, to allow a free passage of air to the assimi- 

 lating gonidial zone. Jatta 2 considers them to be analogous to the lenticels 

 of higher plants and of service in the interchange of gases expelling car- 

 bonic acid and receiving oxygen from the outer atmosphere. It is remarkable 

 that such serviceable organs should have been evolved in so few lichens. 



Fig. 74. Parmelia exasperata Carroll. Ver- 

 tical section of thallus. a, breathing- pores; 

 b, rhizoid. x 60 (after Rosendahl). 



B. BREATHING-PORES 



a. DEFINITE BREATHING-PORES. The cyphellae and pseudocyphellae 

 described above are confined to the under surface of the thallus in those 

 lichens where they occur. Distinct breathing-pores of a totally different 

 structure are present on the upper 

 surface of the tree-lichen, Parmelia 

 aspidota (P. exasperata}, one of the 

 brown-coloured species. They are 

 somewhat thickly scattered as isidia- 

 or cone-like warts over the lichen 

 thallus (Fig. 74) and give it the char- 

 acteristically rough or "exasperate" 

 character. They are direct outgrowths 

 from the thallus, and Zukal 3 , who dis- 

 covered their peculiar nature and func- 

 tion, describes them as being filled with a hyphal tissue, with abundant 

 air-spaces, and in direct communication with the medulla ; gonidia, if 

 present, are confined to the basal part. The cortex covering these minute 

 cones, he further states, is very thin on the top, or often wanting, so that 

 a true pore is formed which, however, is only opened after the cortex else- 

 where has become thick and horny. Rosendahl 4 , who has re-examined these 

 "breathing-pores," finds that in the early stage of their growth, near the 

 margin or younger portion of the thallus, they are entirely covered by the 

 cortex. Later, the hyphae at the top become looser and more frequently 

 septate, and a fine net-work of anastomosing and intricate filaments takes 

 the place of the closely cohering cortical cells. These hyphae are divided 

 into shorter cells, but do not otherwise differ from those of the medulla. 

 Rosendahl was unable to detect an open pore at any stage, though he 

 entirely agrees with Zukal as to the breathing function of these structures. 

 The gonidia of the immediately underlying zone are sparsely arranged and 

 a few of them are found in the lower half of the cone; the hyphae of the 

 medulla can be traced up to the apex. 



Schwendener 1863, p. 169. 



2 Jatta 1889, p. 48. 

 4 Rosendahl 1907. 



3 Zukal 1895, p. 1357. 



S. L. 



