BACTERIA AND PLANTS 89 



or (2) sexual cells, male and female, which in the benign tertian 

 and quartan parasites are rounded, and in the pernicious or 

 malignant tropical form are crescentic in shape. For the con- 

 jugation and further development of these sexual cells, or 

 rather of their products, the blood containing them must be 

 sucked into the " stomach " or mid-gut of a female mosquito of 

 the Genus Anopheles. The cell formed by their union elong- 

 ates and burrows its way through the insect's gut-wall, on 

 the outer side of which it becomes encysted. After a series of 

 complicated changes, during which the cyst enlarges, great 

 numbers as many as 10,000 of minute, elongated, thread- 

 like bodies with pointed ends develop in its interior. These 

 are then liberated into the body-cavity of the mosquito and 

 actively migrate to the salivary glands at the root of its 

 proboscis, ready to be inoculated by the insect into the blood 

 of the next human victim and attack his red blood-corpuscles, 

 and so complete the cycle of development of the parasite. 



As the Allppheles mosquito is thus necessary for the spread 

 of the infection, much good work has been done towards the 

 prevention of the disease by attempting to exterminate this 

 insect-carrier by draining pools and marshes or by pouring 

 paraffin oil into them. The young of the mosquito hatch 

 and develop in such stagnant pools and even in rain- 

 water accumulating in old tin cans, water-barrels, or basins. 

 They require to come to the surface at short intervals to 

 breathe, and the thin scum of oil, spread over the surface of 

 the water, prevents them reaching the air and they are thus 

 asphyxiated. Another well-known precaution is the use of 

 mosquito-nets or curtains to prevent the female insects from 

 biting their human victims during the night, which is the 

 period of their activity. The therapeutic action of quinine for 

 the destruction of the parasites in the human body is, of course, 

 also common knowledge. 



CHAPTER VIII 



BACTERIA AND PLANTS BACTERIA IN THE ARTS AND 

 INDUSTRIES-COMMERCIAL BACTERIOLOGY 



Bacteria and the Nitrogen Cycle Bacterial Diseases 

 of Plants 



Nitrification of the Soil. One of the important chemical 

 elements necessary for plant-life and nutrition is Nitrogen, a 

 large proportion of which has to be provided in an assimilable 



