104 



GENERAL ZOOLOGY 



Stamen 



Pist 



Bubonic plague, or the "Black Death," which swept Eu- 

 rope and Asia during the Middle Ages, killing millions of 

 people, is another insect-borne disease. Rat fleas inoculate 

 the germs into the human body by their bite. One reason 

 why this disease spreads so rapidly is the fact that the 

 germs which produce the plague live in rats and are thus 



carried by ships to all 

 parts of the world. 



Relations between 

 Insects and Flowers. 

 In order to make seed, 

 the flowering plants 

 must have the pollen 

 furnished by the sta- 

 mens (Fig. 64) carried 

 to the pistil of the 

 same kind of plant. 

 The pollen is neces- 

 sary to the fertiliza- 

 tion of the ovule, 

 which later grows into 

 the seed. Continuous 

 pollination of a plant 

 by pollen which it furnishes from its own stamens has been 

 found to be detrimental to the vigor of the seeds. We find in 

 nature many devices to insure fertilization by pollen from an- 

 other plant of the same kind, — that is, by cross-pollination. 

 Some plants have the stamens and pistils so placed in the 

 flower that no pollen can fall from one to the other ; some 

 ripen their stamens and pistils at different times. Many 

 have the stamens and pistils on separate plants, and a great 

 number, though the stamens and pistils are close together, 

 are wholly or partially sterile to their own pollen and "set" 

 their seeds only if the pistils receive pollen from the sta- 



Fig. 64. Diagram of pear flower 



Courtesy of the United States Department 

 of Agriculture 



