70 THE GARDEN OF EARTH 



We have already heard something about living cells, 

 in connection with the lowest forms of vegetable-life, 

 and more must be said later. 



Just now it is enough to observe that a cell is a very 

 minute bag of living liquid. We could not by any 

 possibility see one, unless in a most powerful microscope. 

 But the whole root and all the smaller root-branches are 

 simply made up of masses and masses of such cells 

 as indeed, is the entire plant. They grow and change 

 and alter their shapes, and gain fresh powers, and do 

 different kinds of work, as may be required. 



A word here about the food of plants found in the 

 soil. Some mention was made earlier of "mineral 

 salts," as they are called, which a plant must have 

 if it is to carry out its proper work, and some of the 

 most needful of such " salts " contain soda and potash, 

 iron and phosphorus. These substances, taken in 

 through the roots in liquid form, are worked marvellously 

 into the living substance. 



Among the pressing needs of all plants we are 

 thinking now of the higher kinds, such as grow in field 

 and wood and garden they must have a large amount 

 of carbon, and they must have a large amount of 

 nitrogen. 



The carbon they get out of the air easily enough. 

 And one might expect that they should get their supplies 

 of nitrogen from the same quarter, since enormous 

 quantities of nitrogen float in the atmosphere, much 

 more than of any other gas. 



But, strange to say, they cannot take and use nitrogen 

 straight from the air. They cannot feed upon it at all 

 when it is in what we call a " free " state that is, by 



