INFLUENCE OF TEMPERATURE ON VEGETATION. 657 



Among the changes caused in plants by long-continued depression of temper- 

 ature, one of the most striking is the change in colour of leaves which persist through 

 the winter, originally observed by MohP, and recently more minutely studied by Kraus'-. 

 This change is of two kinds ; the leaves either merely lose their colour and become 

 brownish, yellowish, or rusty brown, as in Taxus, Abies, Pinus, Juniperus, and Buxus ; 

 or turn a decided red on the upper surface, as in Sedum, Sempervivum, Ledum, Ma- 

 honia, Vaccinium, &c. The loss of colour of the first group depends, according to 

 Kraus, on a change in the grains of chlorophyll, which lose their form and definition, 

 a cloudy mass of protoplasm of a reddish brown or brownish yellow colour being 

 formed, while the nucleus remains colourless. These changes are usually more complete 

 in the 'pallissade-cells' on the upper side than in the parenchyma which lies deeper. 

 A spectroscopic examination shows that of the two pigments, a mixture of which forms, 

 according to Kraus, the colouring substance of chlorophyll, the golden-yellow one re- 

 mains unchanged, while the spectrum of the blueish-green substance undergoes a slight 

 change. 



The winter-leaves of the second group, which are coloured red or purplish-brown on 

 the upper side, owe this colour to a rounded hyaline strongly refractive mass lying in 

 the upper part of the pallissade-cells, which appears of a beautiful carmine-red where 

 the leaves are red, but elsewhere of a pale yellow, and consists mainly of tannin. The 

 grains of chlorophyll, intact and of a beautiful green, are all crowded together in the 

 inner end of these cells. In the spongy parenchyma of the mesophyll a colourless or red 

 mass of tannin occurs in the centre of each cell, while the chlorophyll-grains, also intact, 

 are collected in roundish or irregular lumps, sometimes in one place sometimes in 

 several, but always on the sides towards the adjoining cells. In these cases the colour- 

 ing matter of the chlorophyll is unchanged with regard to either of its constituent 

 pigments. The red colouring matter is soluble in water, and cannot be distinguished 

 by spectrum-analysis from the red colouring substances of flowers. 



In all leaves which persist through the winter, and in the green parts of bark, Kraus 

 found that the grains of chlorophyll had removed from the walls to the interior of the 

 cell, and had collected there in lumps (see Sect. 8). When the weather has become 

 sufficiently warm in the spring, the normal condition is restored ; the red colouring sub- 

 stance disappears, and the grains of chlorophyll again take up their normal position on 

 the cell-walls. Kraus shows that the winter change of the leaves depends on the fall of 

 the temperature, since it is restored to the normal state by a simple rise in the temper- 

 ature, whether in the dark or the light. By taking cut branches of box into a warm 

 room when the cold was severe and placing them in water, he found that the proto- 

 plasm of the cells, which had become homogeneous after one or two days, collected on 

 the walls, and then divided into grains (as in the formation of grains of chlorophyll in 

 the dark); the red colouring matter being changed first to a yellowish-green and 

 finally to pure green. After the lapse of three, five, or at most eight days, the walls of 

 the cells became lined with bright green sharply-defined grains of chlorophyll. In 

 Thuja the process required two to three weeks (with me however only a few days). 

 The restoration is therefore rather a slow process ; while, according to Kraus, a single 

 frosty night suffices to bring about the change in the form and colour of the chloro- 

 phyll-grains in the case of Buxus, Sabina, and Thuja. That light has no share in the 

 restoration of the normal condition of the chlorophyll, is shown by the fiict that it 

 takes place also in branches which are kept in a dark room. On the other hand the fact 

 that the parts protected by bemg covered by other leaves show no change of colour 

 would seem to indicate that the whole phenomenon has less to do with the low temper- 

 ature of the air than with the cooling produced by radiation. 



1 Mohl, Vermischte Schriften ; Tubingen, 1845. p. 375. 



■' Kraus, Observations on the winter colouring of evergreen plants ; in the Sitzungsber. der 

 phys.-med. Societut zu Erlangen, Dec 19, 1871, and March 11, 1872. 



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