486 AUTUMNAL COLOURING. 



fact, the rejection of these structures is no sacrifice in reality, since they are 

 only superfluous ballast by which, under certain conditions, the plants may be 

 hampered in their next year's work, and of which they therefore rid themselves 

 most seasonably and suitably. The fall of the leaf may be looked upon, so far, 

 as an excretion of superfluous matter, which, in deciduous plants, occurs only 

 once every year, but is then carried out on a grand scale. To the benefits which 

 this wholesome excretion of waste, formed in the metabolism, affords to individual 

 plants must be added the fact that the fallen leaf reaches the ground with its 

 abundance of lime, decays there, contributes to the formation of humus, which 

 contains calcium nitrate, and so becomes of use to the vegetable kingdom as a 

 whole, as already described in detail. 



The emigration of the useful materials from the leaf-blades to the store-rooms 

 in the interior of the branches and root-stocks must, as a rule, be accomplished 

 fairly quickly; most rapidly, of course, where the period of vegetation during 

 which the foliage-leaves can be active is short, when the leaves are obliged to 

 make use of the "favourable time to the utmost, and where the change of seasons 

 occurs abruptly. The materials withdrawn travel by the same route as in 

 general is taken by the substances normally manufactured in the leaves. The 

 accessories by which the carbohydrates and albumens to be removed are prepared 

 for emigration, might (one would think) be the same in every case. But, just as 

 in one species one kind, and in another a different kind are developed when the 

 leaves are most active, so in different species at the close of the vegetative period, 

 when the great emigration takes place, we have again various accessories, and 

 various despatching and protective agents. In many instances the accessories 

 are colourless, and are not recognizable by the naked eye even when developed 

 in great quantity. It can only be seen that the leaves lose their fresh green 

 on account of the change experienced by the chlorophyll bodies for the purpose 

 of emigration, and that a yellowish tint appears instead of the green colour, which 

 is produced by the already-mentioned yellow granules remaining behind after 

 the departure of the chlorophyll-corpuscles. In many leaves the number of 

 these yellow granules is so small that even the yellow tint is hardly apparent, 

 and these leaves then are a dirty yellowish-white, shrivel up very quickly, and 

 become brown. 



Anthocyanin, however, is produced in many plants during the emigration of 

 the carbohydrates and albuminous materials, and to such an extent, that it becomes 

 plainly visible on the exterior. It appears red in the cell-sap in the presence of 

 acids which occur very regularly as metabolic accessories in the autumn leaves, 

 blue when no acids are present, and violet when the amount of free acids is 

 but small. If there is an abundance of yellow granules together with the acid, 

 red anthocyanin, the leaf assumes an orange colour. Thus the green colour of the 

 foliage changes at the period of the great autumal emigration, sometimes into 

 yellow, or brown, or red, violet or orange, and in this way we have a play of 

 colour exhibiting the greater variety the more numerous are the plant species 



