THALLOPHYTES; FUNGI 77 



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. {joluu iii-)- They are of various forms 

 (Fig. 68), 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 



53. 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. 69, 70, 71). 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 Ijichen is formed of a fungus and 

 an alga, living together so intimately as to appear like a single 



