268 PLANT-LIFE 



transporting agent, as carrying nutrient salts in solution 

 to those tiny chemists who reside in the tissues of plants, 

 and are known as " protoplasts." Transpiration, we 

 have seen, involves the circulation of these salts which 

 go into the plant, and are there combined into com- 

 pounds of an organic character, which promote growth. 

 A green plant has the peculiar virtue of being able to 

 convert inorganic into organic substances in a manner 

 which is apt to bewilder the human chemist. It has 

 been demonstrated that for the satisfactory nutrition of 

 green plants ten elements are essential — these are carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, potas- 

 sium, magnesium, and iron. It has also been shown that, 

 in addition to these essential elements, silicon, chlorine, 

 and sodium, are utilized. It seems that the necessary 

 chemical elements are obtained from compounds, and 

 cannot be used in an uncombined state. Thus, the 

 carbon is taken from carbon dioxide in the atmosphere, 

 and this gas is absorbed by the green parts of plants ; the 

 remaining nine essential elements are gained from various 

 combinations, and absorbed by the root-hairs. Hydro- 

 gen and oxygen are got from water in the soil, water 

 being a combination of these two gases; nitrogen is got 

 from dissolved nitrates, sulphur from sulphates, phos- 

 phorus from phosphates, calcium and magnesium from 

 carbonates, potassium from salts of potassium, and iron 

 from salts of iron. We have elsewhere stated, in our 

 notes on Bacteria (p. 13), that the free nitrogen of the 

 air can be used by plants of the Pea family — i.e., legu- 

 minous plants — through the good offices of certain 

 Bacteria which reside in their roots; but this peculiar 

 arrangement is exceptional, and in the great majority 



