406 DIVISION III. MODE OF LIFE OF THE FUNGI. 



as in Endopyrenium and Catopyrenium. Details on these points will be found in 

 descriptive works and especially in Schwendener's earlier writings. It has been already 

 observed that the medulla is in most cases furnished with broad interstices containing 

 air ; in some fruticose forms, as Cladonia and Thamnolia, there is a broad axile cavity 

 occupying the largest part of the total volume of the thallus. In Usnea a dense axile 

 hyphal strand passes through the middle of the thallus and is surrounded by a loose 

 weft of hyphal branches running to the rind (Fig. 171) ; in Evernia vulpina and E. flavi- 

 cans the loose medulla is traversed by several similar longitudinal strands which 

 approach and separate alternately in an irregular manner. 



The hyphal branches which compose the rind are united together without inter- 

 stices in all cases except in some species of Roccella. They are either unmistakeable 

 hyphal filaments, with the lumina of" their segments at least evidently elongate- 

 cylindrical, though they may not be as long as the cells of the medulla ; or they are 

 composed of short isodiametric rounded prismatic cells, so that the rind often has 

 a very regular and delicate pseudo-parenchymatous structure, as in Parmelia, Physcia 

 (Fig. 172), Endocarpon, Sticta (Fig. 173) and Peltigera. It is only in Bryopogon 

 and Anaptychia ciliaris that the course of the much elongated hyphae of the rind 

 is almost strictly longitudinal even in the mature thallus. In all other cases the hyphae 

 lie in every direction and form an irregularly woven weft or pseudo-parenchyma ; or 

 they are nearly exactly vertical to the surface, as in the pseudo-parenchymatous cell- 

 rows of Endocarpon and Peltigera and in a very beautiful form in tufted branches of 

 Roccella, in which also the lateral connection is somewhat loose and the extremities 

 are free. The single branched hyphae may be clearly distinguished in thin sections 

 especially of Roccella fuciformis, and a slight pressure will isolate them. 



The thickness of the membranes, the breadth of the lumina in the medulla and rind, 

 and the mutual relations of the two layers vary extremely in the different genera and 

 species ; the details will be found in Schwendener. The cell-walls are often unusually 

 thick especially in rind-hyphae with long segments (Usnea, Bryopogon, Sphaerophoron 

 &c.), and from their close connection with one another they seem to form a homogeneous 

 mass, in which the lumina appear as narrow canals. But this homogeneous mass is 

 seen in thin sections, especially when treated with dilute solution of potash or ammonia, 

 to consist of separate obscurely stratified membranes. The structure of these rind-layers 

 has much resemblance to that of some sclerotia (see page 31). 



The differences between the rind of the upper and under side lie in the thickness of 

 the rind-layer, the size and arrangement of the cells, the colour and similar points (see 

 for example Figs. 172, 173). 



The surface of the rind is sometimes covered with a finefe/t of hairs which are small 

 projecting hyphal branches, as on the upper side of Anaptychia ciliaris, Peltigera 

 malacca, P. canina also in the young state, and on the lower side of Sticta and Nephroma. 



Among the wart-like prominences which occur on the upper surface of some foli- 

 aceous and fruticose species, those of Peltigera aphthosa and the smaller ones of Usnea 

 are thickenings or exuberant growths in the rind ; the larger ones in Usnea and the 

 warts and scales of Evernia furfuracea in the matured state are a kind of projections of 

 the thallus, consisting of algal cells inclosed by an air-containing tissue connected with 

 the medulla and invested by the rind. The blackish branched enlargements on 

 the upper side of Sticta fuliginosa and Umbilicaria pustulata are formed of a dense 

 hyphal weft containing Algae surrounded by a single layer of pseudo-parenchymatous 

 rind with a brown membrane. A similar structure has been observed in the wart-like 

 and rod-like excrescences, which often form a thick covering on the surface of many 

 crustaceous Lichens and cause the coralline appearance of the forms known by the name 

 of Isidium ; they require further investigation. 



Peculiar interruptions of the rind occur on the under side of the thallus of Sticta. 

 They are either largish flat spots with a somewhat indistinct outline, as in Sticta pulmo- 

 nacea, or small circumscribed pits, the bottom of which is formed by the exposed 



