THE WORK OF LEAVES 215 



absorption of carbon dioxide. One of the great dan- 

 gers to plants in the neighborhood of factories, etc., 

 is the choking of the stomata with soot a,nd dust. 

 Wherever there is much fine dust, plants suffer in this 

 respect unless frequently washed by rain.^ House - 

 plants are much benefitted by an occasional washing of 

 the leaves with water. 



We can readily understand that it is an advantage 

 to have the stomata placed on the under side of the 

 leaf, since they are then better protected against dust, 

 rain, direct exposure to the sun, etc., and this is the 

 position in which they principally occur. 



Other devices for reducing evaporation that may be 

 studied are the sinking of the stomata in pits (Olean- 

 der) or channels (Cytisus, Broom, etc.), the reduction 

 in the number of the stomata (universal in plants 

 growing in dry situations), the reduction of leaf sur- 

 face (in succulents, such as Hen -and -chickens. Live- 

 forever, Ice Plant, etc.), dispensing with the leaves 

 during a portion of the year (some Switch Plants), or 

 complete and permanent loss of leaves (Cacti) . Obtain 

 any leaves you can of these kinds, and test the amount 

 of evaporation which goes on in them. For this purpose 



1 Plants are sometimes injured when sprayed with oily liquids (to de- 

 stroy insect pests), by the clogging of the stomata with oil. The result is that 

 the leaves turn yellow and begin to drop off soon after the spraying occurs. 



The fumigation of plants with poisonous gases (to destroy insects) must 

 be carried on at night when the stomata are closed. Otherwise the plant is 

 killed. 



