120 THE ART OF CULTURE. 



the hydrogen which is essential to the nourishment of plants, 10 

 presented to them in sufficient quantity. 



When the soil is deficient in moisture, we observe a phenome- 

 non, which appeared quite inexplicable, before we understood 

 the importance of mineral matters, as means of nourishment to 

 plants. We see the leaves close to the soil (those which had 

 been first developed) lose their vitality, shrink and fall off, 

 after becoming yellow, without the apparent action of any inju- 

 rious cause. This phenomenon is not perceived, in this form, in 

 moist years, nor is it observed with evergreens, and only rarely 

 with those plants which throw out long deep roots ; it is observed 

 only in harvest and in winter with perennial plants. 



The cause of this phenomenon is now quite apparent. The 

 matured leaves absorb continually from the air carbonic acid 

 and ammonia, which are converted into the constituents of new 

 leaves, buds, and twigs ; but this conversion cannot be effected 

 without the co-operation of alkalies and of other inorganic sub- 

 stances. When the soil is moist. these are constantly conveyed 

 to the plants, which retain their green color in consequence. 

 But in dry weather, the deficiency of water prevents them being 

 absorbed by the plant; and in consequence of this, they are 

 taken from the plant itself. The mineral ingredients in the juice 

 of the fully formed leaves are abstracted from them, and are 

 employed in the formation of the young sprout ; and when the 

 seeds become developed the vitality of the old leaf is completely 

 destroyed. These withered leaves contain mere traces of soluble 

 salts, while the buds and sprouts are remarkably rich in these 

 ingredients. 



The reverse of this phenomenon is seen in the case of many 

 kitchen plants, when they are supplied with rich manure con- 

 taining an excess of mineral ingredients ; salts are separated 

 from the surface of their leaves, and cover them with a thin 

 white crust. In consequence of these exudations the plant be- 

 comes sickly, the organic activity of the leaves diminishes, the 

 growth of the plant is destroyed, and if this condition lasts, the 

 plant finally dies. These observations are best made on plants 

 with leaves of large dimensions, through which large quantities 

 of water are evaporated. 



