694 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



being present, they readily grow into fresh Lichens. In the majority of Lichens 

 the Algae are arranged in a definite layer or stratum {cf. figs. 392 ^ and 387 ■^); rarely 

 are they distributed through the whole thickness of the thallus, as in the Gelatinous 

 Lichens (Collenia, fig. 393 ^). Apart from these Gelatinous Lichens, the Discolichenes 

 may be divided, according to their form, into Crustaceous, Foliose, and Fruticose 

 Lichens. Crustaceous Lichens include such as are everywhere attached to their 

 substratum (stones, bark of trees, &c.) and cannot be removed without injury. As 

 examples may be mentioned the Lecidias, many of the Lecanoras, &c. The Foliose 

 Lichens are not thus firmly attached; the thallus forms a shield- or ribbon-like 

 expansion, readily separable from its substratum, as, for instance, the well-known 

 orange Lichen Physcia parietina, the green-hued Peltigera canina (PI. XV.), with 



Fig. 393.— Gelatinous Licliens. 

 1 Ephebe Kerneri; x450. - Collema pulposum , nat. size. '-^ Section of thallus of Collema pulposum, with Nostoc as Alga ; x450. 



projecting umber apothecia, common in moist places amongst moss and stones, 

 Unixbilicaria, and many others. In the Fruticose Lichens the plant is only attached 

 at one place, and has a shrub-like, branched appearance. Here are included many 

 elegant and well-known forms, including Usnea harbata, the Old Man's Beard 

 Lichen (PI. XV.), which hangs in tufts and festoons from the branches of trees, 

 and sometimes produces large disc-like apothecia the size of sixpenny pieces. 

 Another of these epiphytic forms is Ramalina reticulata, a Californian form which 

 forms beautiful gray-green, ribbon-like nets. Nor must Cetraria islandica, the 

 Iceland Moss (PI. XV.), and the Cladonias be omitted. The last-named are glaucous 

 and generally erect-growing and branched. They include Cladonia pyxidata, the 

 familiar Cup-moss (PI. XV.), C. rangiferina, the Reindeer-Moss (PI. XV.), and C. 

 coccifera, growing on heaths with its scarlet, and C. macilenta (PI. XV.), with orange 

 apothecia. 



Pyrenolichenes. — In these the asci are contained in perithecia as in the Pyreno- 

 mycetes (cf. p. 678). Otherwise they are in general agreement with the last 

 group, and include crustaceous, foliose, and fruticose forms. There are only a 



