OF THE LEA^^;s. 769 



altogether correspond with those of M. Fremy, whilst the more 

 recent spectroscopic investigations of Prof. Stokes and Mr. H. 

 L. Smith tend to show that chlorophyll is more complex than M. 

 Fremy imagined. 



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 phyllocyanin, and hence that their colour is 

 entirely due to the phylloxanthin, 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 constituents 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 chloro- 

 phyll is composed. The colours of flowers depend on bodies 

 the nature of which is very imperfectly known, though spectro- 

 scopic analysis has done something towards grouping them into 

 series. (See page 28.) The changes in colour which many 

 corollas undergo are supposed to depend on the oxidation of these 

 bodies. Most of the BoraginacecB pass from pink to blue, from 

 their first expansion, till they are fully open ; the garden con- 

 volvulus changes from pink to a fine purple in the same period. 

 Cultivation will effect great changes in this respect, but there is a 

 limit to its influence. The Dahlia and Tulip are naturally yellow, 

 and under cultivation may be made to assume all shades of red, 

 orange, and white, but no tint of blue ; Geranituns and the 

 Hydrangea will take on various shades of blue, purple, red and 

 white, but never a yellow. These facts led De Candolle to divide 

 flowers in this aspect into two series — a xantkic, which has yellow 

 for its base, and a cyanic, which has blue — either of which can be 

 made red or white, but will not assume the basic colour of the 

 other. There seem to be a few exceptions to this rule ; e. g. 

 Myosotis versicolor changes from yellow in the bud to blue in 

 the open corolla, and the Hyacinth is not seldom a pale yellow. 



7. Defoliation, or the Fall of the Leaf. — Leaves are essentially 

 temporary organs; after a certain period, which varies in differ- 

 ent plants, the leaves either wither upon the stem upon which 

 they are placed, as is the case commonly in Monocotyledonous 

 and Acotyledonous plants (see page 178), and also in some 

 Dicotyledonous ones (pa^e 168) ; or they separate from the stem 

 by means of an articulation or joint when they have performed 



