Chapter XIII. 



Other Disease-causing Organisms. 



Bacteria. One does not usually associate bacteria with dis- 

 eases in plants, but they are nevertheless frequently agents of 

 such disease. In recent years many diseases of bacterial 

 origin have been discovered and described. Man's chief inter- 

 est in bacteria is usually centered in the diseases of man which 

 are so largely caused by bacterial action. The bacteria were 

 formerly considered to be fungi but in recent years it has been 

 recognized that they are most closely related to the blue-green 

 algae, though they differ from the latter in important points. 

 They resemble the fungi in their mode of nutrition — for they ob- 

 tain their food in a partially elaborated condition and, with a 

 few exceptions, are unable- to manufacture starch from air, 

 gases and water as do green plants. They are therefore devoid 

 of leaf green just as are the fungi, and thus differ 

 from the blue-green algae. They may possess 

 coloring matters of various kinds but are not as 

 a rule able to utilize these in the conversion of 

 the sun's light to energy in starch manufacture. 

 They differ from the fungi, however, in their 

 method of growth and division, and in these re- 

 spects and in general appearances resemble most 

 closely the blue-green algae. The bacteria, then, 

 may be considered as close relatives of the blue- 

 green algae which have adopted fungus habits 

 of nutrition. 



The forms and sizes of bacteria. The bacteria are all ex- 

 ceedingly minute plants consisting of single cells. They may 

 "be less than one thousandth of a millimeter in length and the 

 largest are seldom more than about ten-thousandths of a milli- 

 meter. It requires, therefore, microscopes with powers of high 



%| 



\ 



ViQ. 90. — Bac- 

 teria of black 

 rot of cabbage 

 (P s e udomonas 

 c a m p e s t ris). 

 They are seen to 

 be tinv cylin- 

 ders. Highly 

 magnified. After 

 H. L. Russell. 



