1881.] 



MICROSCOPICAL JOURNAL. 



lYl 



vertical and horizontal. The verti- 

 cal, or free thallus, is divided into 

 fruticulose and filamentous. The 

 former is a shrub-like aggregation of 

 segments or portions, springing from 

 a centre ; and they are either narrow 

 or broad, round or compressed, sim- 

 ple or branched. Cladonia and Rama- 

 lina are fruticulose forms. The fila- 

 mentous species are more elongated 

 than the fruticulose. They are round 

 and thread-like, more or less clothed 

 with divergent fibrils. Usnea barbata 

 (Fig. 31) shows this form of Lichen. 

 The forms of Lichens just named 

 are those which give the aged and 

 venerable appearance to many of our 

 forest trees, and present them to our 

 sight so shaggy and hoary, as almost 

 to move our pity, while they com- 

 mand our reverence. Wordsworth's 

 '* Thorn " must have been covered 

 with these Lichens. He tells us : — 



" There is a Thorn, it looks so old, 

 In truth you'd find it hard to say 



How it could ever have been young, 

 It looks so old and gray. 



' ' Like rock of stone, it is o'ergrown 

 With lichens to the very top, 



And hung with heavy tufts of moss, 

 A melancholy crop." 



The horizontal thallus embraces 

 the crustaceous and the foliaceous 

 species. The crustaceous (Fig. 32) 

 predominates in number over all 

 forms. It closely adheres to the sur- 

 face on which it grows. In growing, 

 it sometimes assumes a determinate 

 shape, and has a distinct margin 

 when it is called determinate or uni- 

 form. When it spreads itself irregu- 

 larly it is said to be effuse. The 

 thickness of the thallus, and the con- 

 dition of its surface show many modi- 

 fications, which are all considered 

 in the distinguishing of species. 

 The foliaceous or frondose thallus is 

 the most leaf-like of all Lichen 

 growths, and is considered by some 

 as the highest Lichen development. 

 It is a flat, light gray or green expan- 

 sion. Its margin varies in different 

 species. Sometimes they are irregu- 



larly torn into lobes or lacerated, 

 sometimes the lobes are crenate or 

 sinuate or upturned. The margins or 

 lobes are at other times cut into 

 laciniae, these are narrow and linear, 

 or sinuate segments. The foliaceous 

 thallus is most frequently green 

 above, and light or dark colored be- 

 neath. On the under side it is some- 

 times villose, or else has a number of 

 short fibres or bundles of fibres called 

 rhizinae. These have the appearance 

 of small rootlets, but do not act as 

 such ; they simply fix the plant to the 

 matrix or place of growth. Within 

 these general distinctions the Lichens 

 show many features and variations, 

 some of which are characteristic and 

 permanent, others dependent and 

 changing. But these can only be un- 

 derstood by the initiated, yet they 

 are gradually and soon learned by the 

 careful and diligent student. 



The structure of the Lichen thallus 

 is much alike in all forms ; but there 

 is a difference between the gelatinous 

 family and the rest. In the large 

 family of Lichenacei or non-gela- 

 tinous plants the thallus is almost in- 

 variably stratified, and consists of 

 three distinct layers of cellular tissue. 

 But in the family of Collemacei, while 

 there are found the three forms of 

 tissue just referred to, yet they are not 

 stratified. The hyphoid and green 

 cells being mingled together through- 

 out the thallus. In one branch of 

 this family — the Collemas — there is 

 no distinct cortical layer. Another 

 point of this difference between these 

 two families is observed in the ar- 

 rangement of the gonidial cells. In 

 the non-gelatinous plants the green 

 cells are generally free, but in many 

 of the Collemas they are joined to- 

 gether in a moniliform manner, or in 

 the fashion of beads on a string. In 

 some members of the family they are 

 diffused through the thallus, while in 

 a few others they are grouped into 

 small series of cells. 



{^To be continued^ 



