87 



acid, the basis of which is oxygen. In proportion as the oxygen predominates, 

 the leaves become red ; hence the bs lutif ul tints of red and crimson taken by the 

 same leaves in autumn. When the carbon disappears without the nitrogen, 

 the leaves Ijecome yellow. It has been observed that the leaves of plants 

 always turn yellow, red, crimson, or violet, and never blue, and this corresponds 

 with the above theory, as the carbon, which is dark, is carried out of the leaves 

 by the descending sap, and its place partially supplied with oxygen. . . . The 

 Lime and other trees which abound in mucilage or gluten, further corroborate this 

 theory in having their decaying leaves yellow. . . . The colouring matter 

 is not in the sap, which is either colourless, or faintly tinged with yellow ; thus, 

 while the stem consists chiefly of cellular tissue, it remains as green as the 

 leaves." 



To the remark as to the Lime-tree, I may add that the Acer campestre 

 (Common Maple) in almost every hedgerow, furnishes a parallel in the changes 

 of its leaves, which are further remarkable for the ruddy tint which they assume 

 in spring. 



The presence of iron, manganese, and other mineral substances, and the 

 peculiar character of the soil, are also important elements in the produc- 

 tion of colour. We have all seen, in the cultivation of garden flowers, the im- 

 mense changes, not alone in colour, but in form and size, which are effected by 

 the artificial alteration of the soil. 



The main cause of colour, however, must be admitted to te solar light. 

 Plants found in mines generally differ from their congeners of the same species 

 in the absence, total or comparative, of the colours which mark those grown in 

 the sunlight. The recorded exceptions to this rule are not numerous, and seem to 

 be confined to some of the lower cliisses of vegetable existence, viz., mosses and 

 algae, with an occasional case of ferns. I do not remember reading any instance 

 in which the plant belonged to the phanerogamous, or flowering division of the 

 vegetable kingdom. 



The colours of young flowers change by fixed laws, just as we see the hues of 

 fruits or seed vessels alter as they approach maturity. It has been found that 

 these changes may be all classified thus : — 



1. Red changes into white, blue, or maroon. The Digfitalis purpurea (Fox- 

 glove) commences with white flowers, which become red, deepening into purple, 

 and then fading into white again. Under some circumstances the Anagallis 

 arvensis (Pimpernel) sends forth blue instead of scarlet flowers, but that is an 

 accidental change rather than a progressive one. The hue usually alters as it 

 fades to a maroon. The bud of the Myosotis palustris ( Water Forget-me-not) 

 which is tipped with reddish purple, becomes white, and then blue as it expands ; 

 while the Convolvulus arvensis (Small Bind-weei) fades from flesh colour to 

 white. 



2. Blue passes into white. In the I'olemonium (Greek Valerian) blue and 

 white flowers will be found side by side upon plants produced by the spread of 

 the same mother stock ; but the change appears to be one of seminal \ ariety. 



