DEVELOPMENT OF PLANTS 141 



scum that appears upon cooked foods and soups, if allowed to 

 stand undisturbed for a day or more (Fig. 91, A, 3). Conn 

 estimates that a single bacterium, if allowed to develop under 

 perfect conditions, would produce in three days a mass of bac- 

 teria weighing 4,700 tons. When the conditions are unfavorable 

 for growth many species of bacteria have another form of re- 

 production and form small bodies, called spores, within the cell 

 (Fig. 91, A, D). These spores are exceedingly resistant to un- 

 favorable conditions but germinate readily, producing bacteria 

 when conditions are again suitable for growth. Some spores will 

 stand boiling for more than one hour. This peculiarity of the 

 spore, while of great advantage to the bacteria, renders them very 

 difficult to exterminate. 



(b) Exclusion of Bacteria. — The various devices for preserv- 

 ing fruits and meats and sterilizing utensils and instruments in 

 surgery is based upon some method of killing these organisms 

 and preventing the entrance of others. This is the purpose of 

 boiling and canning foods and of salting or drying fruits and 

 meats. The boiling, if thorough, effectually sterilizes while the 

 canning excludes all organisms and the material will remain un- 

 changed indefinitely. Smoking or drying, with addition of salt 

 in the case of meat and of sugar in fruits, is effectual since 

 bacteria flourish only in the presence of moisture. This is the 

 reason that seeds and other structures are not speedily destroyed 

 by bacteria. Seeds are naturally dry and protected by coats that 

 tend to exclude the moisture for some time. Sugar is used as 

 a preservative because bacteria cannot endure over 40 or 50 per 

 cent, of it. Salt for the same reason is useful in preserving 

 meat because it is injurious to bacterial life and smoking also 

 introduces volatile products that prevent their growth. 



(c) Economic Importance of Bacteria. — Insignificant and 

 simple as are the forms and structures of bacteria it is safe to 

 say that no plants are so vitally related to our welfare. A small 

 number of species construct necessary food products ; others form 

 certain mineral compounds ; some remove the remains of ani- 

 mals and plants through putrefaction and decay ; and others pro- 



