14 THE FUNGI WHICH CAUSE PLANT DISEASE 



1 mm. in diameter. (Fig. 5.) 500 to reach across it; and 392,700 

 placed side by side or 785,400 if placed on end to cover its area, and 

 about 500,000,000 to fill a cube the edge of which is 1 millimeter, 

 making no allowance for lost space of the interstices. Considerably 

 more than 500,000,000,000 bacteria of this size would find room 

 enough to move about in a space of one cubic centimeter. 



The typical mode of increase among bacteria — ^the only mode 

 except among the sheath bacteria — is by fission or direct divi- 

 sion of one cell, the mother cell, into two, the daughter cells. 

 Fig. 6. The rapidity with which fission can proceed depends of 

 course upon conditions of environment, ranging from no growth 

 at all, due to cold, lack of nutriment, presence of inhibiting sub- 

 stance, to a maximum that varies with the species. For bacteria 



QOOD 00 QQ 



Fio. 6. — Diagram illustrating the fission of bacteria, bacilli and cocci. 

 After Novy. 



in general under very favorable surroundings, with proper tem- 

 perature and abundance of food, from 20 to 40 minutes may be 

 reckoned as a generation. In 24 hours, with the divisions once 

 each hour, the progeny of one germ will be 16,777,216; with 

 divisions each 30 minutes it will be (16,777,216)*- 



If cell division be in one direction only and the resulting daugh- 

 ter cells remain undisturbed, a thread-like row results. If cell 

 division be in two planes, and the resulting cells adhere in groups, 

 tablets of 8, 16, and 64 will occur frequently. If the division be 

 in three planes and their cells adhere, packets result. 



The structure of the bacterium cell owing to its minuteness 

 is yet very incompletely known. The most enduring portion of. 

 the vegetative cell is the cell wall. This is surrounded by a layer, 

 the capsule and bears the fiagella. The number of the flagella 

 and their position varies in different species. Some species have 

 none, some one, two, or many. They may be at the ends, polar, 



