CHAPTER IV. 



SOHE PLANTS THAT DO NOT MAKE THEIR OWN LIVING. 



This chapter deals with parasitic and saprophytic plants of all 

 ranks. Of course any real study of bacteria is not to be thought of 

 in elementary work ; but although the subject must be treated purely 

 in an information way, its natural interest and practical bearings seem 

 to justify giving it some attention. Bacteria are very minute unicel- 

 lular plants. It would take about half a million of some kinds to 

 cover the dot of an " i " on this page. The plants reproduce by cell 

 division with almost incredible rapidity. It has been estimated that, 

 under favorable circumstances, one plant can give rise to sixteen 

 millions in eight hours. This fact explains the rapid progress of 

 some germ diseases, the cholera for instance. Some bacteria form 

 spores, and many of them offer considerable resistance to adverse 

 conditions. Some can survive even freezing or boiling temperatures 

 and very long periods of dessication. The universal presence of bac- 

 teria is much less alarming because of the fact that many species are 

 harmless. 



Bacteria that have not access to free oxygen in the air, have the 

 power of obtaining it by breaking up fluid compounds that contain 

 oxygen. In this way they may rob the blood of oxygen and bring 

 about other decompositions that are supposed to be the causes of some 

 diseases. Nature seems to have two methods for counteracting the 

 effects of these injurious bacteria. Animal bodies contain cells that 

 can digest bacteria ; they also contain, it is said, certain substances 

 termed defensive proteids that can destroy bacteria or their products. 

 So immunity from certain diseases can be secured by one of two arti- 

 ficial methods. The bacteria that cause the disease can be cultivated 

 until there results what is called an attenuated virus ; inoculation 

 with this virus leads to a very mild form of the disease and prevents 

 a virulent attack. The other method is to introduce the defensive 

 proteids into the system. 



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