82 MORPHOLOGY 



3. FOLIOSE LICHENS 



A. DEVELOPMENT OF FOLIOSE THALLUS 



The larger leafy lichens are occasionally monophyllous and attached at 

 a central point as in Umbilicaria, but mostly they are broken up into lobes 

 which are either imbricate and crowded, or represent the dividing and 

 branching of the expanding thallus at the circumference. They are hori- 

 zontal spreading structures, with marginal and apical growth. The several 

 tissues of the squamule are repeated in the foliose thallus, but further pro- 

 vision is made to meet the requirements of the larger organism. There is the 

 greater development of cortical tissue, especially on the lower surface, and 

 the more abundant formation of rhizoidal organs to attach the large flat 

 fronds to the support. There are also various adaptations to secure the aera- 

 tion of the internal tissues 1 . 



B. CORTICAL TISSUES 



Schwendener 2 was the first who, with the improved microscope, made 

 a systematic study of the minute structure of lichens. He examined typical 

 species in genera of widely different groups and described their anatomy in 

 detail. The most variable and perhaps the most important of the tissues 

 of lichens is the cortex, which is most fully developed in the larger thalli, and 

 as the same type of cortical structures recurs in lichens widely different in 

 affinity as well as in form, it seems well to group together here the ascertained 

 facts about these covering layers. 



a. TYPES OF CORTICAL STRUCTURE. Zukal 3 , and more recently 

 Hue 4 , have made independent studies in the comparative morphology of 

 the thallus and have given particular attention to the different varieties 

 of cortex. They each find that the variations come under a definite series 

 of types. Zukal recognized five of these: 



1. Pseudoparenchymatous (plectenchyma): by frequent septation of 

 regularly arranged hyphae and by coalescence a kind of continuous cell- 

 structure is formed. 



2. Palisade cells: the outer elongate ends of the hyphae lie close 

 together in a direction at right angles to the surface of the thallus and form 

 a coherent row of parallel cells. 



3. Fibrous: the cortical hyphae lie in strands of fine filaments parallel 

 with the surface of the thallus. 



4. Intricate: hyphae confusedly interwoven and becoming dark in 

 colour form the lower cortex of some foliose lichens. 



1 See p. 126. 2 Schwendener 1860, 1863 and 1868. 3 Zukal 1895, p. 1305. 4 Hue 1906. 



