MORPHOLOGY AND PHYSIOLOGY. 45 



tissues of the foliose thallus : 1. Upper or inner corti- 

 cal layer ; 2. Upper or inner algal layer ; 3. Medullary 

 layer ; 4. Lower or outer algal layer ; 5. Lower or 

 outer cortical layer. Rhizoids are not present ; cilia 

 may occur, as in Cetraria islandica. 



In the cylindrical fruticose thalli, all the tissue-ele- 

 ments are arranged in a cylindrical fasliion. The cen- 

 tral portion may be hollow, as in Cladonia and Pilo- 

 phoron, or solid, as in Usnea. This arrangement of 

 tissue-elements is in accordance with mechanical prin- 

 ciples, giving the required support to the usually large 

 thalli. (Plates II and IV.) 



n. THE APOTHECIA. 



The apothecia are the spore-bearing structures de- 

 veloped in or upon the thallus. They are structurally 

 very similar to the spore-})roducing organs of fungi 

 (Ascomycetes, sac-fungi) but functionally, the similar- 

 ity is not so close. Among fungi, each spore is cap- 

 able of developing into a mature spore-producing 

 plant; that is, it is the true reproductive element of 

 the fungus. In lichens it is essentially different; here 

 the spore cannot develop into a new spore-produc- 

 ing plant for the simple reason that the spore 

 and its product represents only a part of the lichen- 

 autonomy ; that is, the germinating spore must be asso- 

 ciated with the requisite algae before a new lichen can 

 develop. This difference is very important and should 

 be kept in mind when comparing lichens and fungi. 

 The spores of lichens are functionally degenerate, re- 

 productive organs derived from the fungal ancestor 



