THE PLANT AND ITS PARTS. 7 



The stem, exceptiug the runners, is entirely under- 

 ground in Ojfhilu cer'niccf. Its use is to develop leaves 

 and flowers as well as to produce the little bulbs for 

 multiplying the plant. 



The leaf is one of the most important parts of a 

 plant, for by means of it the plant can ''digest" its 

 mineral food, which has been partly drawn up by the 

 roots, dissolved in water ; but it also takes in and lives 

 upon the impure air (called carhonie acid gas) which we 

 breathe out of our lungs, and as lone? as the sunlic^ht 

 lasts, leaves and other green parts of plants continue to 

 purify bad air by breathing out in exchange the pure 

 air (oxi/gcji (jas) whicli we require to breathe in. 



This process of AssimUalion, as it is called, consists 

 in decomposing carbonic acid, which is made up of the 

 two elements Carbon and Oxijfjcn, of which the plant 

 retains the carbon, uniting it with the elements of 

 water, and so makes starch. The oxygen is, as stated, 

 set free into the atmosphere. If some very delicate 

 leaf of a water-plant or of a moss which has been 

 exposed to bright sunlight for some hours be brushed 

 over with a camel's-liair brush dipped in tincture of 

 iodine, the leaf will turn to a violet colour. This 

 indicates the presence of starch. If a slice of potato 

 be treated in the same way, it turns of a violet colour 

 too, as starch has been stored up in the cells to a very 

 large extent iu potatoes. 



There is another important use of leaves, called 



