316 IOWA ACADEMY OF SCIENCE 



depends upon them. Plant cells which contain chlorophyl break 

 up the carbonic acid gas with which they come in contact, re- 

 taining the carbon, one of its elements, and sending back the 

 oxygen into the air. Under the influence of sunlight they com- 

 bine the carbon with the' oxygen and hydrogen of the water 

 from the roots into the new chemical compounds ; in which 

 nitrogen and the earthy constituents mentioned above, that is 

 to say the food material which reaches the tree through the 

 roots and leaves, are first digested in the body and are then 

 sent to all living parts of the roots, stem and crow r n. Some of 

 this food is stored away until the proper moment arrives. "Wood 

 is made up chiefly of carbon, oxygen and hydrogen. When per- 

 fectly dry about half its weight is carbon and half oxygen and 

 hydrogen in almost the same proportion as water. It contains 

 about one part in a hundred by weight of earthy constituents. 

 The nitrogen and w T ater taken up by the roots were originally 

 in the air before they reached the ground. It is true therefore 

 that when wood is burned those parts which came from the air 

 go back into it in the form of gas, while those which came from 

 the soil remain behind in the form of ashes. Besides giving out 

 oxygen through the leaves to the air they breathe through the 

 minute openings in the bark. This breathing goes on day and 

 night and consequently more carbonic acid gas is taken into 

 the tree than is given out and the surplus carbon is used in 

 growing. The addition of new material or the foundation of 

 growth is deposited in a thin coat over the whole tree between 

 the wood and the bark. There are two layers of this coat sep- 

 arated by a third one of tender tissues, and the outer or cam- 

 bium layer forms new bark. Wood is chiefly made up of very 

 small tubes or cells of various kinds which have special use in 

 the life of a tree. Some conduct water from the roots to the 

 crown, some store away digested food, and others merely 

 strengthen the structure of the wood and hold it together ; but 

 in each case some of the cells have thick walls and small open- 

 ings and others wide openings and thin walls. Consequently at 

 first the tree makes thin walls itself and wide openings through 

 which water can rise rapidly to the ends of the branches ; later 

 on when the demand of water is not so great and there is plenty 

 of digested food to supply building material the cells formed 

 are narrow and thick walled. Thus the summer growth in 

 wood is heavier, stronger, darker in color than spring wood. 

 Hldora. 



