28 THE WORLD WE LIVE IN 



Materials found in tree. Our experiments have shown us that 

 elements may be separated from compounds, and elements may be 

 built up into compounds. In a living tree similar processes are con- 

 tinually going on. Compounds containing the elements sulphur, 

 potassium, iron, etc., are taken from the earth, dissolved in water. 

 Nitrogen, which is an absolute necessity for building living 

 material, is taken from the earth in the form of very complicated 

 compounds which usually come from the decaying bodies of plants 

 and animals and are found in the black soil we call humus. Oxygen 

 comes from the air and is taken into the plant through breathing 

 holes in the leaves. All together, the tree is a wonderful laboratory, 

 for out of these raw materials it builds foods and from these foods 

 it builds its own wonderful structure. The leaves not only make 

 food in the sunlight, but they digest and circulate it to all parts 

 of the plant. This may be done in darkness as well as in light. 



The table on the opposite page gives us the characteristics of 

 the most common elements that are found in the tree and its en- 

 vironment. It will also tell us how to identify each of these ele- 

 ments by testing for those properties by which they are known, 

 where they are found, and what use they have in nature. 



Self-Testing Exercise 



Matter is anything that has (1) or occupies (2). 



It may be in the form of a (3), a (4), or a (5). 



Energy is (6) to do (7). One form of energy may be 



(8) into another, as is seen in making (9) by means of 



water power. Energy may be stored in a substance as (10) 



energy, but when released to do work, it is (11) energy. 



Both (12) and (13) things are made up of 



(14) elements. There are over (15) in all. The commonest 



ones found in living things are (16), (17), 



(18), and (19). Elements combine in (20) propor- 

 tions to form (21), as two parts of (22) and one 



part of (23) form water. When anything (24) with 



oxygen, we say (25) is taking place. If an organic substance 



is (26) or oxidized, it gives off (27) (28). 



If (29) turns (30), when used as a test for this gas we 



know that ........ (31) (32) is present, 



