288 



THE ESSENTIALS OF AGRICULTURE 



suckers it pierces these plants and extracts their juices for 

 food. The plant attacked becomes stunted and the yield of clover 

 and alfalfa is seriously reduced when fields are badly infested 

 with dodder. Mistletoe (Fig. 136), well known from its wide- 

 spread use at Christmas time, lives on the branches of trees. 

 Its suckers penetrate the bark and extract its food. Mistletoe 

 is a common pest on ornamental and forest trees throughout 



the South and Southwest. 



376. Bacteria. Bacteria 

 are exceedingly abundant, 

 occurring in the air, water, 

 soil, and in bodies of ani- 

 mals. They are invisible to 

 the naked eye. They may 

 live singly or in groups in 

 the substance on which they 

 feed. They reproduce by 

 dividing in two, the whole 

 process often taking less than 

 half an hour, so that under 

 favorable conditions a small 

 number of these plants may 

 produce hundreds of thou- 

 sands in a few hours. 



Most bacteria are either 

 harmless or beneficial, but 



some cause disease. Tuberculosis and typhoid fever are due to 

 the growth of harmful bacteria in the human body ; and the 

 wilt of melons and cucumbers, the wilt of tomatoes and potatoes, 

 the well-known pear blight (Fig. 137), and the crown gall com- 

 mon to apples, roses, alfalfa, and many other plants are well- 

 known bacterial diseases. 



377. Fungi. Fungi are also exceedingly abundant and are 

 found living on decaying organic matter (saprophytes), such as leaf 

 mold, rotting wood, and manure, and as parasites on living plants. 

 They do not have true root systems, but masses of rootlike threads 



FIG. 136. Mistletoe a plant parasite 



The leaves of the oak tree are young and small, 



and the bunches of mistletoe conspicuous. 



(Photograph by Dr. W. L. Bray) 



