348 CHEMISTRY. 



vator. If the soil be destitute of potash, neither turnips 

 nor grape-vines will grow well in it. If it be destitute of 

 lime, it will not answer for clover or pease. Liebig divides 

 all cultivated plants into three classes, according to the 

 chemical character of their ashes: I. Potash plants, the 

 ashes of which contain more than half their weight of salts, 

 having alkaline bases (potassium and sodium), soluble in 

 cold water. The beet, mangel-wurzel, and turnip belong 

 to this class. 2. Lime plants, the ingredients of which are 

 salts of lime and magnesia, soluble in acids. In this class 

 we have clover, beans, pease, tobacco, etc. 3. Silica plants, 

 in which silica predominates in the ashes. Wheat, barley, 

 rye, and oats are in this class. 



484. Water in Plants. In the processes of vegetation 

 water not only furnishes some of the material, but it is the 

 common carrier, as we may say, of all the other materials. 

 What is taken in by the roots and the leaves is carried to 

 all parts of the plant by the water. In doing this work 

 the water courses through the plant in larger quantity than 

 is commonly supposed. We can get some idea of this by 

 looking at the amount which is exhaled from the leaves of 

 plants into the air. Some investigations have been made 

 on this point. It was found that from the leaves of a sin- 

 gle cabbage there passed into the air nearly a quart of wa- 

 ter in twenty-four hours. With this great exhalation from 

 plants there must be a large amount of water passing up 

 from the earth through them in a rapid but quiet circula- 

 tion. 



485. Annual Changes in Plants. When annual plants 

 have stored up in their seeds a sufficient quantity of starch 

 and albuminous substance for the germs of a new race of 

 plants their work is done, and they fall to decay. But in 

 perennial plants, such as shrubs, fruit and forest trees, after 

 their fruit or seed has ripened, the woody fibre which has 



