VEGETABLE KINGDOM DIAGRAM 10, 171 



RESPIRATION AND NUTRITION OP PLANTS. 



The root, stalk, and leaves serve for the respiration, absorption 

 and nutrition 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 air, and 

 breathe out carbonic acid. But the breathing of plants during the 

 day 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. 



The 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 or yellow 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 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 Floiucr 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 not always the case, 

 as they are sometimes very small and inconspicuous. The nettle, 



