IS2 OUTLINES OF PLANT LIFE. 



are important medicines. They occur in the seeds, bark, or leaves, and 

 are gotten rid of when these are dropped. 



212. Summary. — The elements required for the nutrition 

 of plants may be determined by analysis. The chief com- 

 pounds are found to be water and carbon compounds. Water 

 and the mineral salts dissolved in it are absorbed by land 

 plants from the soil by means of root-hairs. Since the water 

 tends to become equally distributed through the soil the 

 roots draw their supply of solutions not only from parts with 

 which they are in contact, but also from more distant regions. 

 They are also able to dissolve certain solids. The water ab- 

 sorbed moves into the stele, often under pressure, and is 

 carried, by unknown forces, to the leaves, through the wood 

 strands. It is constantly evaporating from the leaves, which 

 regulate the amount in various ways. 



Foods are required to repair waste and provide for growth. 

 Colorless plants must absorb these from solution ; if the foods 

 are not already soluble they must be made so by digestion. 

 The foods they use are carbon compounds which have been 

 made by some other living being. Green plants can use 

 ready-made food, or, if suitably illuminated, they can make 

 foods out of carbon dioxid and water, with small quantities 

 of mineral salts. The carbon dioxid is absorbed from air by 

 the leaves. Light furnishes the energy for building up the 

 simple substances into carbohydrates. Proteid foods are 

 also made by working into the carbohydrates additional 

 necessary elements. Foods may be used at once or stored, 

 usually in solid forms, for a longer or shorter time. When 

 needed they are digested and transferred. 



Respiration of plants is exactly like respiration of animals. 

 Its purpose is to release energy stored in the living proto- 

 plasm to enable it to work, i.e. to grow, move, etc. Respi- 

 ration consists in the absorption of oxygen, the decomposi- 

 tion of protoplasm, and the excretion of carbon dioxid and 



