AUTUMNAL COLOURING. 485 



differ essentially according as the leaves of the plant continue active through 

 one, or through several vegetative periods, i.e. according as the leaves are deciduous 

 or lasting but one year, or evergreen, that is to say, lasting for two or more 

 years. Evergreen leaves are so organized in all those regions whose climate 

 necessitates a temporary suspension of vital activity, that they may be able to 

 survive the periods of drought or frost of one or even of several years without 

 injury. Before they enter upon their summer sleep in regions of summer 

 drought, or their winter trance in regions with cold winters, alterations occur 

 in their cells, which, in the main, terminate in the diminution of the water 

 contents and the formation of substances which will not be altered by the pre- 

 vailing frost or dryness. In regions where we have a winter sleep, the chlorophyll- 

 granules assume a yellowish-brown or brownish-red colour, and adhere together 

 in clumps, which withdraw as far as possible from the surface of the leaf, 

 travelling down to the floor of the palisade-cells and occupying their lower ends. 

 These alterations are only slightly apparent outwardly in perennial leaves pre- 

 paring for their winter period of rest; the only thing one notices is that the 

 leaves, which in summer are a vivid green, exhibit a darker green, or incline 

 to brown or yellow; which change of colour is observed to the greatest extent 

 in Thuja, Cryptomeria, Sequoia, Chamcecyparis, Libocedrus, and generally in most 

 evergreen conifers. 



The changes which are accomplished in leaves lasting only one year, at the 

 onset of the summer drought or winter cold, are much deeper rooted and obvious. 

 These leaves are not clad so as to be able to defy the drought or frost, and 

 are therefore thrown oflf at the commencement of the unfavourable period. 

 If these leaves were to fall without further ceremony, all the substances in the 

 tissues of the leaves, whose production entailed a considerable amount of work, 

 would be entirely lost. But it is part of the economy of plants that such a 

 waste is carefully guarded against. Before the leaves are detached, the carbo- 

 hydrates and albuminous materials, in general everything which is of use to the 

 plant, is conveyed from the leaf -blades into the woody branches or subterranean 

 root-stocks, and there deposited in places where they find a safe resting-place, 

 and can survive the drought of summer or cold of winter unharmed. In this 

 way the plant suffers only the slightest loss in the materials manufactured by 

 it in the preceding vegetative period; for the leaves from which everything 

 useful has been transported into the stem-structures now form nothing more 

 than a dead framework, and their cell-chambers contain only small yellow 

 granules, together with crystals of calcium oxalate, which cannot be employed 

 further, and are of no more use (see fig. 123 ^). The shining yellow granules, 

 which are found in the cells of fallen leaves, and to which is due the yellow 

 colouring of autumn foliage, are to be regarded as the ultimate useless residue 

 after the withdrawal of the transformed chlorophyll-corpuscles. The crystals 

 of calcium oxalate have arisen in the foimation of albumens by the decomposition 

 of nitric and sulphuric acids. Both of them can be sacrificed. As a matter of 



