230 NATURE STUDY AND AGRICULTURE 



layer between the wood and bark. This layer is called 

 the cambium. This is the part of the twig in which the 

 growth of new wood takes place, and these bacteria are so 

 numerous that they use up most of the nourishment of the 

 twig. As a result the twig dies and, of course, the leaves 

 must die too. The best way known to prevent the spread 

 of blight is to cut off every twig that is affected and burn it. 

 There are many other kinds of bacteria besides those 

 that cause pear blight. They are so small that it is not 

 practicable to attempt to study them by observation. 

 Many of them are not more than one twenty-five thousandth 

 of an inch long. It is worth while to know that while these 

 plants are small they are among the most important living 

 organisms. They are one-celled plants and reproduce by 

 cell division instead of by spores. That is, each cell 

 divides into two parts, each of which becomes a mature cell. 

 This division continues, making the multiplication very 

 rapid. Like the spores of mold, these cells float about in 

 the air or in water. Some cause human disease, such as 

 typhoid fever, diphtheria, etc. Others are beneficial to 

 man, such as those that cause decay of vegetable and 

 animal matter in the soil. Without these we could not 

 have any humus in the soil. The bacteria that live on the 

 roots of leguminous plants are beneficial. We shall see 

 in our study of clover just what they do for us. 



Farmers' Bulletin: The Grain Smuts, No. 75. 



