30 STRUCTURAL AND PHYSIOLOGICAL BOTANY. 



231. All leaves ultimately fall off; evergreen leaves later 

 than others. 



232. The mode in which leaves are arranged within their 

 bud is called vernation, or gemmation. 



233. Leaves have, under particular circumstances, the 

 power of producing leaf-buds from their margin (185) ; Ex. 

 Bryophyllum, Malaxis paludosa, and proliferous Ferns. 



VII. FOOD AND SECRETIONS. 



234. Plants are nourished by the absorption of food from 

 the air and earth, in consequence of which they grow, and 

 produce their peculiar secretions. 



235. The growth of plants is very rapid ; that of the leaves 

 is such that they often acquire six or seven times their original 

 weight per hour. 



236. The food of plants always consists of carbonic acid, 

 nitrogen, and water, and also of various mineral matters, 

 chiefly alkaline, the nature of which varies according to 

 species. 



237. Boots have the power of absorbing most substances in 

 a fluid or gaseous form, even although their extremities are 

 unbroken. 



It appears probable that when plants are incapable of imbibing certain sub- 

 stances, such as strontian, there is no isomorphism between their ordinary 

 mineral constituents and those they reject. Thus, lime and magnesia, which 

 plants will indifferently absorb, are isomorphous ; but between them and 

 strontian, which they will not absorb, no isomorphism exists. Daubeny. 



238. Carbon is obtained by plants in the form of carbonic 

 acid, derived from the atmosphere, or generated in soil by the 

 decay of vegetable matter. 



239. Hydrogen is obtained principally by the decomposition 

 of water, and is assimilated along with carbonic acid, while 

 the oxygen of the water is liberated. 



240. Nitrogen can only be obtained by plants in the form 

 of ammonia. The nitrogen of the atmosphere cannot be the 

 source of supply, because it cannot be made to enter into com- 

 bination with any element except oxygen, even by the em- 

 ployment of the most powerful chemical means. 



241. Ammonia exists in every part of plants, in the roots, in 

 the stem, and in all blossoms and fruits in an unripe condition. 



