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 
