u6 



FIRST GROUP. THALLOPHYTES. 



attached to the substratum at one point only and by a narrow base, and grow upwards 

 from it with a branching shrubby habit. The branches of the thallus are either flat and 



FIG. 75. A. Usnea barbata, a fruticose Lichen, natural size. B piece of the thallus of Sticta pultnonacea, a 

 foliaceous Lichen, natural size, seen from below ; a apothecia, f the disk in A by which the Lichen is attached to 

 the bark of a tree. 



ribbon-like, very similar to the lobes of many foliaceous lichens, or they are slender and 



cylindrical (Fig. 75, A). In Cladonia 

 and Stereocaulon we have not so much 

 intermediate forms between the folia- 

 ceous and fruticose thallus, as a com- 

 bination of the two, where there is at 

 first a small foliaceous expansion, and 

 then a cup-shaped or branched and 

 shrub-like thallus proceeding from it. 



The thallus of Lichens may be dried 

 till it is ready to be ground into powder 

 without losing its vitality ; if it is then 

 soaked in water, it usually acquires the 

 consistence of leather, and is tough 

 and elastically flexible ; but there are 

 a number of genera, remarkable also 

 on other accounts, in which the thallus 

 in the soaked state is slippery and ge- 

 latinous ; these gelatinous lichens form 

 cushion-like masses with a wavy sur- 

 face, and in growth approach some- 

 times the fruticose and sometimes the 

 foliaceous lichens ; Collema is a typical 

 lichen of this kind (Fig. 74). 



The arrangement of the gonidia and 

 the hyphae in a thallus may be such, 



FIG. 76. Sticta fuliginosa., transverse section through the 

 foliaceous thallus, magn. 500 times ; o rind- (epidermal) layer of 

 the upper side, u that of the under side, rr rhizines or attaching 

 structures which spring from the epidermal layer, -m the medullary 

 layer, the filaments of which are to be seen in longitudinal and 

 transverse section; the upper and under rind also consist of hyphae, 

 but their short cells have much broader lumina and are connected 

 together without interstices, forming a pseudo-parenchyma ; g the 

 gonidia with their light-green protoplasm shaded dark ; each gela- 

 tinous envelope encloses several gonidia formed by division. 



that these two elementary forms seem 

 to be distributed in it uniformly and 

 in equal proportions, as in Fig. 77 ; in 



