HOW PLANTS BENEFIT AND HARM MANKIND 175 



layer of cells in the leaf of the host. Among the useful plants preyed 

 upon by this group of fungi are the plum, cherry, and peach trees. (The 

 diseases known as black knot and peach curl are thus caused.) Other 

 sac fungi are the morels and truffles, the downy mildews, blue and green 

 molds, and many other forms. One important member of this group is 

 the tiny parasite found on rye and other grains, which gives us the drug 

 ergot- 



Problem XXTII. A study of bacteria and of some of their 

 relations to man. (Laboratory Manual, Prob. XJCIII.) 

 (a) Conditions of growth. 

 (ZO Some relations to man. 

 (e) Some methods of fighting harmful bacteria. 



Bacteria. The bacteria are found in the earth, the water, 

 and the air. " Anywhere but not everywhere," as one writer 

 has put it. They swarm in stale milk, in impure water, in the liv- 

 ing bodies of plants and animals, and in 

 any decaying material. These tiny plants, 

 " man's invisible friends and foes," are of 

 such importance to mankind that thou- 

 sands of scientists devote their whole lives 

 to their study, and a science called bac- 

 teriology has been named after them. 



Size and Form. In size, bacteria are 

 the most minute plants known. A bac- 

 terium of average size is about T<TO o" of an 

 inch in length, and perhaps 25000 of an 

 inch in diameter. Some species are much 

 larger, others smaller. A common spheri- 

 cal form is FoVo of an inch in diameter. It 

 will mean more to us, perhaps, if we re- 

 member that several millions of bacteria of 

 average size may be placed within the area 

 formed in this letter o. Three well-defined 

 forms of bacteria are recognized : a spheri- 

 cal form called a coccus, a rod-shaped 

 bacterium, the bacillus, and a spiral form, the spirillum. Some 

 bacteria are capable of movement when living in a fluid. Such 



Bacteria, highly magnified : 

 a, the germ of typhoid 

 fever, stained to show the 

 cilia, little threads of liv- 

 ing matter by means of 

 which locomotion is ac- 

 complished ; 6, a spiral 

 ciliated form ; c, a rod- 

 shaped form, in chains ; 

 d, a spherical form. a, 

 6, from Engler and Prantl. 



