120 BOTANY FOR BEGINNERS CHAP. 



corks. Mix five solutions for water culture, and number the bottles 

 containing them from one to five. 



(1) Let the first be the normal solution given on p. 118. 



(2) Leave out the potassium nitrate from the second solution. 



(3) Mix the third solution without the iron chloride. 



(4) From the fourth leave out magnesium sulphate. 



(5) In the fifth substitute sodium nitrate for the potassium nitrate. 

 Measure the plants from time to time. Note 



(i) How the plant grows in the first solution ; the growth will be 

 normal. 



(ii) The plant which is grown without the potassium nitrate is stunted 

 in growth. 



(iii) The plant grown without iron is not green. Wash a leaf with a 

 weak solution of iron chloride ; it will turn green. 



(iv) The plant grown without magnesium sulphate is very stunted in 

 its growth. 



(v) The plant grown without potassium nitrate, but for which sodium 

 nitrate is substituted, is also abnormal in its growth. This shows that 

 sodium cannot take the place of potassium. 



The Food of Plants. Plants can only make use of 

 soluble food, t.e., they can only take in food in solution. The 

 essential elements can only be assimilated when they are united 

 to form compounds. 



Carbon. In the experiments in water culture, the solutions 

 contained no carbon. But if the mature plants at the close of 

 the experiment be submitted to analysis, half their dry weight 

 will be found to be Carbon. Where did the carbon come from ? 

 Not from the solutions, but from the atmosphere which sur- 

 rounds the green parts of the plants. The atmosphere may 

 be regarded as a mixture of gases in the following proportion : 



Nitrogen 79*00 ^ 



Oxygen 20*96 ] Parts by Volume. 



Carbon dioxide 0*04 J 



1 00*00 



The green parts of plants are alone able to take in carbon 

 dioxide and decompose it into carbon and oxygen. That green 

 plants give out oxygen can be shown by placing a few leaves 

 of any water plant (Elodea or Water Cress will do) in water, 

 and exposing them to bright sunlight, when bubbles of gas will 

 be given off. If these bubbles of gas are collected and exam- 

 ined, they are found to consist of oxygen. If a green plant, or 



