Schwendener[345] 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[346], and more recently Hue[347], 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. These four types, Zukal finds, are practically without interstices in the tissue and form a perfect protection against excessive transpiration. He adds yet another form: 5. =A cortex formed of hyphae with dark-coloured swollen cells=, which is not a protection against transpiration. It occurs among lower crustaceous forms. Hue has summed up the different varieties under four types, but as he has omitted the “fibrous” cortex, we arrive again at five different kinds of cortical formation, though they do not exactly correspond to those of Zukal. A definite name is given to each type: 1. =Intricate=: an intricate dense layer of gelatinous-walled hyphae, branching in all directions, but not coalescent (Fig. 44). This rather unusual type of cortex occurs in _Sphaerophorus_ and _Stereocaulon_, both of which have an upright rigid thallus (fruticose). [Illustration: Fig. 44. _Sphaerophorus coralloides_ Pers. Transverse section of cortex and gonidial layer near the growing point of a frond × 600.] [Illustration: Fig. 45. _Roccella fuciformis_ DC. Transverse section of cortex near the growing point of a frond × 600.]