BACTERIA 



261 



as usually, this takes place on a large scale, the surface of the 

 thallus acquires a powdery appearance. The individual granules 

 (so-called soredia, Fig. 142, A), being distributed by the wind, 

 afford a rapid means of propagation of the Lichen. 



The bright colours of many Lichens are due to the presence 

 of pigments which are remarkable for their " fast " character, 

 and have been employed in dyeing. The well-known chemical 

 indicator litmus is obtained from species of Roccella. Iceland 

 Moss (Cetraria islandica, Fig. 143), which forms a slimy fluid 

 when boiled with 

 water, is used as an 

 invalid food. 



The Bacteria con- 

 stitute an extremely 

 important group of 

 Thallophyta whose re- 

 lationships are very 

 obscure. The majority 

 of species are colour- 

 less unicellular plants 

 of extremely small 

 size. The larger speci- 

 mens are not more 

 than i/iooth mm. in 

 length and about one- 

 tenth as wide, whilst 



the globular forms average i/ioooth mm. in diameter. Some 

 are so small as to be almost, if not quite, invisible under the 

 highest powers of the microscope. They occur in almost every 

 possible situation, and live under the most varied conditions. 

 They are not killed by cold, and some can survive for many 

 months at the temperature of liquid air. Whilst most die if 

 heated to about 50 C., a few (thermophilic Bacteria) live in 

 fermenting hay and manure heaps which, owing to their activity, 

 may attain a temperature of 70 C. 1 Many Bacteria also in- 

 habit the waters of hot springs. 



1 Hence the employment of manure for hot-beds, etc, 



FIG. 143. Iceland Moss (Cetraria islandica), 

 part of a large specimen, about natural 

 size. 



