THE INTER-RELATIONS OF PLANTS AND MAN 125 



groups of living things which has seriously threatened the 

 survival of various races of mankind. In Part Five, this relation 

 of plants to man is considered more in detail. 



In the same part is taken up the warfare which plants wage 

 among themselves. A sunlit meadow or a quiet forest gives an 

 impression of peace and goodwill, suggesting that plants are far 

 removed from the life-and-death struggles which characterize 

 animal lives. But underneath its placid exterior the plant world 

 is rife with the struggle for existence. The greatest battles are 

 those fought between the green plants and the hosts of colorless 



Fig. 81. — Poisonous plants include poison ivy and the Amanita mushroom. 



fungi waiting to parasitize them. Microscopic fungus spores, 

 carried through the air, penetrate the stomates of a leaf or fissures 

 in the bark, and once within the host plant, germinate into 

 invisible parasites which will live their entire lives at the expense 

 of the photosynthetic host. Such fungus-caused diseases are 

 numerous, sometimes threatening extinction of a species as 

 happened in the blight which attacked the eastern chestnut 

 trees. Plant diseases, in fact, are even more numerous than those 

 of human beings; but the average person is not aware of them 

 except when the host plant is a valuable crop. It is in these 

 instances that the warfare between fungi and green plants 

 becomes important to man. Often he must step in to lend aid to 

 his green allies lest they become vanquished in the struggle. The 



