132 MORPHOLOGY 



C. General Aeration of the Thallus 



Definite structures adapted to secure the aeration of the thallus in a 

 limited number of lichens have been described above. These are the breathing- 

 pores of Parmelia exasperata and the cyphellae and pseudocyphellae of the 

 Stictaceae, with which also may be perhaps included the circumscribed 

 breaks in the under cortex in some members of that family. 



Though lichens are composed of two actively growing organisms, the 

 symbiotic plant increases very slowly. The absorption of water and mineral 

 salts must in many instances be of the scantiest and the formation of carbo- 

 hydrates by the deep-seated chlorophyll cells of correspondingly small 

 amount. .Active aeration seems therefore uncalled for though by no means 

 excluded, and there are many indirect channels by which air can penetrate 

 to the deeper tissues. 



In crustaceous forms, whether corticate or not, the thallus is often deeply 

 seamed and cracked into areolae, and thus is easily pervious to water and 

 air. The growing edges and growing points are also everywhere more or 

 less loose and open to the atmosphere. In the larger foliose and fruticose 

 lichens, the soredia that burst an opening in the thallus, and the cracks 

 that are so frequent a feature of the upper cortex, all permit of gaseous 

 interchange. The apical growing point of fruticose lichens is thin and porous, 

 and in many of them the ribs and veins of their channelled surfaces entail 

 a straining of the cortical tissue that results in the formation of thinner 

 permeable areas. Zukal' devoted special attention to the question of aeration, 

 and he finds evidence of air-passages through empty spermogonia and through 

 the small round holes that are constant in the upper surface of certain foliose 

 species. He claims also to have proved a system of air-canals right through 

 the thallus of the gelatinous Collemaceae. Though his proof in this instance 

 is somewhat unconvincing, he establishes the abundant presence of air in 

 the massively developed hypothecium of Collema fruits. He found that the 

 carpogonial complex of hyphae was always well supplied with air, and that 

 caused him to view with favour the suggestion that the function of the 

 trichogyne is to provide an air-passage. In foliose lichens, the under surface 

 is frequently non-corticate, in whole or in part; or the cortex becomes 

 seamed and scarred with increasing expansion, the growth in the lower 

 layers failing to keep pace with that of the overlying tissues, as in Umbili- 

 caria pustulata. 



It is unquestionable that the interior of the thallus of most lichens con- 

 tains abundant empty spaces between the loose-lying hyphae, and that these 

 spaces are filled with air. 



1 Zukal 1895, p. 1348. 



