80 ELEMENTARY BOTANY. 



amounts of the several elements. The different species there- 

 fore have, at least to some extent, a selective power, and con- 

 sequently they absorb the different food elements in different 

 amounts. When one kind of crop is grown continuously for 

 years in the same soil the yield usually decreases — due doubt- 

 less to the partial exhaustion of some one or more of the food 

 elements or at least of the soluble compounds which furnish 

 the same. But another kind of crop would perhaps draw 

 more heavily on other elements and so by alternating or " by 

 rotation " the soil is less quickly exhausted, time being given 

 in the interim for the accumulation of the various soluble 

 compounds that furnish the needed foods. 



12. The mineral or inorganic food taken into the plant must 

 undergo chemical changes to become vegetable fabric. The 

 conditions under which these take place, and the processes 

 are but partially understood. But it is known that the carbon- 

 dioxide and water are decomposed, and the elements are re- 

 combined to form carbon compounds. Starch is the first 

 visible product, though it is probable that some other carbohy- 

 drate (perhaps glucose) is really the first in the series. The 

 process involving the above changes takes place only in the 

 chlorophyll-body in the presence of sunlight (or electric lightj. 

 It is called photo-synthesis} 



Tie a piece of Elodea or other water plant to a glass rod and place it 

 (with the lower cut end of the stem uppermost) in a beaker or vessel of 

 spring water, or water containing carbon dioxide. When placed in strong 

 sunlight a stream of bubbles will escape ; fewer will be seen if the light is 

 less intense, and none will appear in the absence of light. To show that this 

 escaping gas is oxygen, have in the vessel of water a number of shoots of 

 the water-plant covered by an inverted funnel over which is placed a test- 

 tube filled with water. The gas (oxygen) which collects in this test-tube can 

 be tested with a glowing splinter. 



13. Starch is formed in the leaf or other green parts only 

 during the day. It becomes during the night dissolved 



' It has until recently been called assimilation, but this word can be most 

 advantageously used in the sense in which it is employed in animal physi- 

 ology. 



