182 NATURE STUDY AND AGRICULTURE 
simple: they are spherical, oblong, or cylindrical bodies 
having no roots, leaves, or flowers. They multiply in the 
simplest manner that we could imagine. Each individual 
simply splits in two, and in a very short time these parts 
are full-grown and ready to divide again. In some kinds 
this division takes place every 20 or 30 minutes so that two 
or three generations may be produced in an hour. Bac- 
teria are found almost everywhere, but we are here con- 
cerned only with those kinds that grow in the tissues of living 
DISEASE-PRODUCING BACTERIA 
A, the tuberculosis bacterium; B, the diphtheria bacterium; C, the bacillus 
of typhoid fever; D, the bacillus that causes lockjaw; some of the cells are 
forming spores. 
plants. An example of a plant disease due to bacteria is the 
fire-blight of pear and apple trees. 
Fungi are much larger than bacteria, and with some of the 
most conspicuous kinds nearly all people are familiar. The 
largest fungi are the mushrooms, toadstools, and puffballs, but 
most of these live upon decaying animal and vegetable matter, 
and we are now interested only in those that infest living 
plants and make them sick. Fungi have no stems, roots, 
or leaves. Their main part usually consists of a mass of 
whitish threads that are sometimes finer than a spider’s web. 
These threads lengthen with great rapidity and may extend 
