OF THE LEAVES. 745 



seem, that phyllocyanine is not an immediate principle, but that 

 it is formed by the alteration of phylloxantheine. The more 

 recent experiments of M. Filhol do not, however, altogether 

 correspond with those of M. Fremy. 



The autumnal tints of leaves, which are generally some shades 

 of yellow, brown, or red, are commonly regarded as due to 

 varying degrees of oxidation of the chlorophyll which their cells 

 contain. The experiments of M. Fremy show, that the yellow 

 leaves of autumn contain no phyllocyanine, and hence that their 

 colour is entirely due to the phylloxanthine, either in its original 

 condition or in an altered state. 



When leaves are of some other colour than green, the different 

 colours are produced, either, by an alteration of chlorophyll or 

 of one of the principles of which it is formed ; or in consequence 

 of the presence of some other colouring principle. 



Variegation in leaves must be regarded as a diseased condition 

 of the cells of which they are composed; it is commonly produced 

 by hybridization, grafting, differences of climate, soil, &c. The 

 variegated tints are due, either to the presence of air in some of 

 the cells ; or more commonly, to an alteration of the chlorophyll 

 of certain cells, or of one of the substances of which the chloro- 

 phyll is composed. 



7. Defoliation, or the Fall of the Leaf. — After a certain period, 

 which varies in different plants, the leaves either die upon the 

 stem upon which they are placed before they fall, as is the case 

 commonly in those of Monocotyledonous and Acotyledonous 

 plants (see page 184), and also in some Dicotyledonous ones 

 (page 174); or they separate from the stem by means of an 

 articulation or joint when they have performed their active func- 

 tions, or even sometimes when quite green. In the former case 

 the leaves are non- articulated ; in the latter articulated. In the 

 trees of this and other temperate climates the leaves commonly 

 fall off the same year in which they are developed, that is, before 

 the winter months ; and in those of warm and tropical regions 

 the fall of the leaf often takes place at the dry season. The leaves 

 of other plants, however, generally remain for two or more years. 

 In the former case they are annual or deciduous, and in the latter 

 persistent. The fall of the leaf is commonly termed defoliation. 



The cause or causes which lead to the fall of the leaf are by 

 no means well understood. The opinion commonly entertained 

 is this : the cells of the leaf contain various inorganic matters 

 dissolved in their liquid contents, and therefore, as exhalation of 

 watery matters proceeds, the membrane constituting their walls, 

 gradually become incrusted by the deposit of earthy matters 

 which are left behind, until ultimately, the tissues of the leaf 

 become choked up -with such deposits, and are no longer able to 

 perform their proper functions. The leaf then begins to dry 

 up, its attachment to the stem becomes weakened, and it ulti- 



