328 INTRODUCTION" TO BOTANY 



307. Air and temperature. In the earlier sections of this 

 book the air relations of plants have been presented in the 

 various discussions of photosynthesis and of plants of differ- 

 ent regions. Under most circumstances the air is likely to 

 contain enough of the proper gases, and not too much of harm- 

 ful ones, to enable plants to thrive. In large cities and in the 

 vicinity of manufacturing establishments, especially smelters, 

 harmful gases are present in quantities that often kill plant 

 life. For fifteen miles or more about certain copper smelters 

 all plant life has been killed. In commercial greenhouses and 

 in homes the plants may exist in the presence of illuminat- 

 ing gas, but 1 part of illuminating gas in 80,000 parts of air 

 will prevent carnations from flowering, and other plants are 

 prevented from flowering and often killed by such gases. 



In the vicinity of cement manufacturing plants the cement 

 gets into the stomata of the leaves, becomes hard and closes 

 the stomata, and eventually results in killing all plant life. 



308. Prevention of plant diseases. The development of plant 

 industries has tremendously stimulated the study of plant dis- 

 eases. Many volumes have been written upon the topic, and 

 every agricultural experiment station issues bulletins from 

 time to time, to advise people about the latest discoveries in 

 regard to the nature of important diseases and the means of 

 preventing them. The ways in which these diseases operate 

 are various. They may use the food material made by the 

 host plant, as is supposed to be true in case of wheat and oat 

 rusts ; they may also consume or supplant important tissues, 

 as when corn and oat smut occupy the grains of their hosts ; 

 they may stop the vascular tissues, as is the case with the 

 bacteria that produce the brown wilt of cabbage and other 

 related plants ; or they may excrete substances that are 

 poisonous to the host plants. 



Diseases due to animals such as plant lice or aphids, scale 

 insects, and larger insects are extremely destructive. The 

 intricate nature of such diseases may be shown by using as 

 an illustration the aphids that often produce serious injury 



