THE COLOURS OF LEAVES IN WINTER. 



321 



are less profound. The green spores of many Algae, for instance, become deep 

 red, without any essential changes of the plastic material occurring at the same 

 time : with the commencement of the new period of vegetation, the red colour- 

 ing matter disappears, the chlorophyll again begins to assimilate, and the cells 

 to grow. In some Phanerogams, the leaves which persist through the winter 

 behave similarly in many respects : here, however, the intense cooling in winter, 

 particularly by radiation, co-operates. After Mohl had remarked these processes 

 in 1845, they were more closely studied by Kraus in 187 1 and 1872. These 

 winter-leaves either become simply discoloured and . brownish, yellow, or red-brown, 

 as in Taxus, Pinus, Abies, Juniperus, and Biixus, or of a decided red on the 

 upper side, as in Sedum, Sempervivum, Mahonia, and Vaccinium. The dis- 

 colouration of the first group depends, according to Kraus, on an alteration of 

 the chlorophyll : the chlorophyll-grains lose their shape and distinctness, and form 

 an ill-defined cloudy mass of protoplasm of a red-brown or brown-yellow colour, 

 while the nucleus of the cell remains colourless. The winter-leaves which are 

 coloured red or purple-brown on the upper side owe this colour, according to 

 Kraus, to a rounded, hyaline, strongly refractive mass, in which tannin predominates, 

 situated in the upper part of the palisade-cells : the chlorophyll-grains themselves 

 are intact, and of a fine green, and are all crowded at the inner ends of these cells. 

 In the spongy parenchyma of the mesophyll, a red or colourless globule of tannin 

 is found in the centre of each cell, and the chlorophyll-grains, likewise intact, 

 are collected into lumps on the sides, sometimes at one, sometimes at several places. 

 In all winter-leaves, as well as in the green parts of the cortex, Kraus found that 

 the chlorophyll-grains had passed from the walls to the interior of the cells, and 

 were there aggregated in clumps. In the spring, in sufficiently warm weather, 

 the normal condition is restored : the red colouring matter disappears, and the 

 chlorophyll -grains again assume their normal distribution on the cell-walls. If 

 branches of the first-named group of plants are cut off in the cold of winter and 

 brought into a warm chamber, they assume their normal green colour after a few 

 days, even in the dark. 



It has already been mentioned that the green colouring matter of the chlo- 

 rophyll-grains can be extracted by strong alcohol, aether, chloroform, and also 

 by fat oils, the colourless protoplasmic matrix remaining behind. Chemists 

 have taken much trouble to investigate the chemical nature of this extracted 

 colouring matter, without having obtained so far any satisfactory result. It 

 is not even established whether the chlorophyll colouring matter, for the 

 production of which small quantities of iron are necessary, as I have shown, 

 itself contains iron. The view proposed by the French chemist Frdmy, that 

 the green colouring matter is a mixture of a yellow and a blue, has not been 

 confirmed, since it is scarcely doubtful that the apparent breaking up of the 

 colouring matter into two others is simply a chemical decomposition \ It is not 

 necessary, however, to go further into this discussion; since the views so far 

 proposed as to the chemical nature of the chlorophyll colouring matter have no 

 reference to the physiological functions of chlorophyll. The same is true of the 



' See my 'Lehrbuch der Bot' IV. Aufl. 1S74, p. 731. 

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