THALLOPHYTES : FUNGI 293 
They are the smallest known living organisms, the one- 
celled form which develops on cooked potatoes, bread, milk, 
meat, etc., forming a blood-red stain, having a diameter of 
but 0.0005 mm. (-g^^o in.). They are of various forms 
(Fig. 268), as Coccus forms, single spherical cells ; Bacterium 
forms, short rod-shaped cells ; Bacillus forms, longer rod- 
shaped cells ; Leptothrix forms, simple filaments ; Spirillum 
forms, spiral filaments, etc. 
They multiply by cell division with wonderful rapidity, 
and also form resting spores for preservation and distri- 
bution. They occur everywhere in the air, in the water, 
in the soil, in the bodies of plants and animals ; many of 
them harmless, many of them useful, many of them dan- 
gerous. 
They are intimately concerned with fermentation and 
decay, inducing such changes as the souring of fruit juices, 
milk, etc., and the development of pus in wounds. What 
is called antiseptic surgery is the use of various means to 
exclude bacteria and so prevent inflammation and decay. 
The pathogenic forms that is, those which induce dis- 
eases of plants and animals are of great importance, and 
means of making them harmless or destroying them are 
being searched for constantly. They are the causes of such 
diseases as pear-blight and peach-yellows among plants, and 
such human diseases as tuberculosis, cholera, diphtheria, 
typhoid fever, etc. 
LICHENS 
194. General character. Lichens are abundant every- 
where, forming various colored splotches on tree-trunks, 
rocks, old boards, etc., and growing also upon the ground 
(Figs. 269, 270, 271). They have a general greenish-gray 
color, but brighter colors may also be observed. 
The great interest connected with Lichens is that they are 
not single plants, but each Lichen is formed of a fungus and 
an alga, living together so intimately as to appear like a single 
