seem absurdly small ; for we are constantly engaged in supplying 

 fertilizers to the soil, and never seem to trouble ourselves about 

 this important substance carbon. It was an interesting fact that 

 the carbon went off as a gas when the plant was burned in air. 

 The carbon did not gooff alone, l^ut it went off in connection with 

 oxygen, and in a form called carbon dioxid gas, CO^. 



2. The air contains a small percentage of carbon dioxid, but 

 oxygen and nitrogen are the abiindant elements. — The green plant 

 must get its carbon from the air. In other words, much of the 

 solid matter of the plant comes from one of the gases in the 

 air. Carbon dioxid is only about four-tenths of one per cent in 

 the air. It would, however, be very disastrous to animal life if 

 this percentage were much increased. 



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 combustion, that is, if the torch is 

 extinguished, it usuall}' means that no wise man would care to 

 breathe such air. The air of a closed school-room often contains 

 far too much of this gas along with little particles of solid carbon. 



3. The carbon dioxid of the air readily diffuses into the leaves 

 and other green parts of the plant. — The leaf is delicate in texture, 

 and often the air can enter directly into the leaf tissues. There 

 are, however, special inlets provided for the diffusion of gases 

 into the leaves and other green parts. These inlets consist of 

 numerous pores (stomates, or stomata) which are especially 

 abundant on the under surfaces of the leaves. The apple leaf 

 contains about one hundred thousand of these pores to each 

 square inch of the under surface. Through these stomates the 

 outside air enters into the air spaces of the plant ; and finally 

 into the little cells containing the living matter. 



4. The greeyi color of leaves is due to a substance called chloro- 

 phyll. — Purchase at the drug store about a gill of the poison, 

 wood alcohol. Secure a leaf of geranium, or of any convenient 

 plant which has been exposed to sunlight for a few hours, and 

 put it in a white cup with sufficient alcohol to cover the leaf. 

 Place the cup on the stove where it is not hot enough for the 

 alcohol to take fire. After a time the coloring matter is all dis- 



