^56 



Medical Botany. 



this order were included by Linnoeus 

 under one genus, Lichen ; but Dr. E. 

 Acharius, a learned Swedish botanist, 

 has formed the genus into a natural 

 order, consisting of three suborders, 

 or sections, and forty-two genera. 

 The plants of this order generally con- 

 sist of a crustaceous expansion, de- 

 nominated by Linnaeus the frond, and 

 by Acharius the thallus, and commonly 

 covered with apothecaj or tubercles, 

 which are not very conspicuous unless 

 magnified (a). The plant is found 

 sparingly on the maritime rocks of the 

 south of England, particularly in Portland Island, and on the sea rocks of 

 Cape Verd and tlie Canary Islands. In France it is used to a considerable 

 extent, in the southern provinces, for dying silk ; and there are now large 

 manufactures of it in London and Liverpool. " The Lichen is imported to 

 us as it is gathered, and litmus is prepared from it thus : — The plant is 

 first dried, cleansed, and pulverised in a mill, like the oil mill. The 

 powder is then thrown into a trough, with one half of its weight of pearl- 

 ash ; is moistened with a little human urine, and then allowed to ferment. 

 This fermentation is kept up for some time by successive additions of urine, 

 till the colour of the materials first changes to a purplish red, then to blue. 

 In this state it is mixed with a third of its weight of very good potash, and 

 spread upon deep wooden trays till dry. A quantity of chalk is added at 

 last, apparently for the mere purpose of increasing its weight." It may here 

 be remarked, that another species of Roccella, R. fusiformis, is reported 

 to vie in richness of colouring matter with the common Orchal, while the 

 plant attains to a much larger size. This species, like the former, occurs 

 sparingly on the sea rocks of the south of Europe ; but it is said to abound 

 in the East Indies, especially on the shores of Sumatra, and might deserve 

 the notice of some of our enterprising countrymen. It has no medicinal 

 properties ; but litmus is used in chemistry as the most delicate test, either 

 by staining paper with it, or by infusing it in water, which will presently 

 turn red by acids, and have the blue colour restored by an alkali. 



Cetraria {cetra, a buckler; receptacle) islandica {fig. 77.), Iceland Lichen 

 Cetraria, or Eryngo-leaved Liverwort. A native of the mountainous heaths 

 and woods in the alpine parts of Bri- 

 tain. The late Sir J.E.Smith gathered 

 it on the Pentland Hills near Edin- 

 burgh, on Ben Lomond, and in various 

 parts of Scotland. In Sweden, Nor- 

 way, and Iceland, the plant is much 

 more luxuriant than in Britain. The 

 plant here seldom exceeds 3 in. in 

 height, and is erect and bushy. " The 

 fronds form loose, elegant tufts ; they 

 are membranous, somewhat cartila- 

 ginous, channelled below, variously 

 sinuated,andlobed; the lobes beingir- 

 regularly divided, notched, and fringed 

 at the margin with hairs. The surface 

 of the whole vegetable is smooth, shining, of a rich chestnut brown colour, 

 with a green tinge when wet, paler underneath. In such plants as grow in 

 the shade, the colour is a pale brownish green. The fructification, or shields, 

 are dark chestnut, large, polished, flat, without any border, growing close to 

 the upper surface of the fronds {a a a, and magnified b). By analysis, 100 



