422 IjHSSOns with plants 



and place the plant with its ball of remaining 

 earth (which is probably larger in amount than 

 that extracted by the plant) in a horizontal posi- 

 tion, and balance it upon the finger. Are the stem 

 and top heavier than the root and earth com- 

 bined? The plant has not only consumed some 

 of the earth, but it has derived food from other 

 sources. 



538. There are but two sources from which 

 this extra food can be obtained, — from the water 

 applied to the roots, and from the atmosphere. 

 The pupil cannot determine if the plant lives 

 from these two sources of nourishment, except that 

 he knows that the plant dies in the absence of 

 either water or air. 



539. He must be told, then, how the plant 

 lives. It absorbs substances in solution by means 

 of its roots. (Consider the root -hairs.) Water 

 must be present, or the soil materials cannot 

 be dissolved. The water, therefore, acts as a car-* 



, rier of food ; but the plant also uses the ele- 

 ments of water itself as foods. The elements 

 which the plant takes in through its roots are 

 many, but those required in large quantity are 

 potassium, sodium, calcium, phosphorus, silicon ; 

 others are necessary. 



540. The plant absorbs gases by its leaves, 

 chiefly carbon dioxid, which consists of carbon 



