VEGETABLE KINGDOM DIAGRAM 10 171 



EESPIRATION AND NUTRITION OF PLANTS. 



The root, stalk and leaves serve for the respiration and nutri- 

 tion of plants ; for plants like animals, breathe and feed. They 

 breathe air by their leaves ; and they feed on the water which 

 they pump up by their roots, and which always contains a 

 great number of salts and other substances in solution. We 

 have seen that in breathing, animals absorb the oxygen of the 

 ail-, and breathe out carbonic acid. But the breathing of plants 

 is just the reverse; plants absorb the carbonic acid of the air, 

 and reject the oxygen. Therefore plants purify the air we breathe 

 and it is partly for this reason that it is more healthy to live in 

 the country than in town. 



Nevertheless, cut plants in a room may become disagreeable, 

 on account of their odour, which makes some persons ill ; and it 

 is injurious to sleep in a room which contains bunches of flowers. 



T^he water absorbed by the roots is changed into sap, and this 

 rises from the roots to the leaves. It is enough to cut a branch 

 of some trees in spring to see the sap flow in abundance. The 

 sap is often sweet. 



But sap is not always the only liquid which flows in vegetable 

 tissues. When we break the stalk of a spurge, of a poppy, or a 

 dandelion, a milk-white oryellc^y fluid exudes. This liquid has a- 

 very acrid taste, and is very different from the sap ; it is called 

 the milk of the plant. Some of these are used in pharmacy and 

 in the arts ; opium, india-rubber, and gutta-percha are the milky 

 juices of the poppy, and of some trees which grow in warm 

 countries. This milk, though generally more or less acrid and 

 poisonous, is not always so ; and the milk of some tropical trees 

 is sweet and nourishing. 



The flower is that part of the plant destined to produce the 

 fruit and seed from which a fresh plant will spring. Many 

 flowers are beautifully coloured, but this is riot always the case ; 

 they are sometimes very small and inconspicuous. The nettle, 



