INFLUENCE OF TEMPERATURE ON VEGETATION. 
735 
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, Semper'vi'uum, Ledum, Mabonia, 
Vaccinium, &c. The loss of colour of the first group depends, according to Kraus, on 
a change in the chlorophyll-granules, which lose their form and definition, a cloudy 
mass of protoplasm of a reddish brown or brownish yellow colour being formed, while 
the nucleus of the cell remains colourless. These changes are usually more complete 
in the 'pallisade 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 siibstance of chlorophyll, the golden-yellow one 
remains unchanged, while the spectrum of the bluish-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 pallisade-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 
chlorophyll-granules, 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-granules, 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 colouring 
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 chlorophyll-granules 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 chlorophyll-granules 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 tempera- 
ture, whether in the dark or the fight. By taking cut branches of Box into a warm 
room M^hen 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 chlorophyll-granules 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 chlorophyll-granules. 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-granules 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 fact 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 being covered by other leaves show no change of colour 
would seem to indicate that the whole phenomenon has less to do with the low tempera- 
ture of the air than with the cooling produced by radiation. 
4. Convenient contrivances for observing the action of particular higher or lower 
temperatures on plants or parts of plants of considerable size are easily arranged ^ 
It is more difficult to expose microscopic objects to a particular higher or lower 
temperature in such a manner that it can easily and certainly be observed, and that 
the temperature of the object is also that indicated by the thermometer, or nearly so. 
^ Sec Sachs, Handb. der Exp.-Phys. pp. 64, 66. 
