LEAVES— FUNCTION OR WORK 93 



without free access of air, or smothered, as in a charcoal 

 pit. A mass of charcoal remains, almost as large as the 

 body of the plant. Charcoal is almost pure carbon, the ash 

 present being so small in proportion to the large amount 

 of carbon that we look on the ash as an impurity. Nearly- 

 half of the dry substance of a tree is carbon. Carbon 

 goes off as a gas when the plant is burned in air. It does 

 not go off alone, but in combination with oxygen in the 

 form of carbon dioxid gas, COj. 



The green plant secures its carbon from the air. In 

 other words, much of the solid matter of the plant comes 

 from one of the gases of the air. By volume, carbon dioxid 

 forms only a very small fraction of i per cent of the air. 

 It would be very disastrous to animal Hfe, however, if this 

 percentage were much increased, for it excludes the life- 

 giving oxygen. Carbon dioxid is often called "foul gas." 

 It may accumulate in old wells, and an experienced person 

 will not descend into such wells until they have been tested 

 with a torch. If the air in the well will not support com- 

 bustion, ■ — • that is, if the torch is extinguished, — it usually 

 means that carbon dioxid has drained into the place. The 

 air of a closed schoolroom often contains far too much of 

 this gas, along with little solid particles of waste matters. 

 Carbon dioxid is often known as carbonic acid gas. 



Appropriation of the Carbon. — The carbon dioxid of the 

 air readily diffuses itself into the leaves and other greeji 

 parts of the plant. The leaf is delicate in texture, and when 

 very young the air can diffuse directly into the tissues. 

 The stomates, however, are the special inlets adapted for 

 the admission of gases into the leaves and other green 

 parts. Through these stomates, or diffusion-pores, the out- 

 side air enters into the air-spaces of the plant, and is finally 

 absorbed by the Httle cells containing the living matter. 



