These changes of tint sometimes appear to depend on the temperature. Thus, the white lilac was obtained by horticulturists under the influence of a temperature of between 30° and 35° C. We can not, however, affirm that spontaneous races with white flowers originated in the same way as the white lilac. It will be enough to point out a few facts that may contribute to the guidance of persons who are seeking to learn the origin of these colored varieties. The _Papava alpinum_ has a very stable variety with yellow flowers, which, according to Focke, has been observed in the polar regions, while the white varieties have been seen in Switzerland. The cultivation of the same species at Giessen, Germany, has made it possible to obtain specimens with white flowers by metamorphosis from specimens with yellow flowers, but it is impossible to say whether or not heat is the agent that produces the changes in these cases. The experiments of MM. Schübela and Bonnier have shown that flowers become darker without changing their color in high regions and in those near the pole; but this phenomenon is one of light and not of color. Be their origin what it may, these white and colored forms have remarkable fixedness. It will be observed that black does not figure in the table of the classification of colors given above. Absolute black, in fact, does not exist in any flower. If some parts appear black, it is only because their tint is excessively dark. The black of the petals of _Pelargonium triste_ and of the bean is yellow, and that of the _Orchis nigra_ is a brown. Apparent blacks are, moreover, extremely rare. The gamut of the reds is much more varied than that of other colors. The reds of the xanthic series are generally more lively-hued, carnation or flame-colored; those of the cyanic series present tints more nearly approaching violet. These two reds may furthermore give rose-colors, but a little skill will divine their origin. The rose of the hydrangea inclines to blue, while that of the rose tends rather toward yellow. Blue colors are the most variable, and readily pass to violet and red, but most frequently to white. The most tenacious hues are those of yellow, and we might affirm that the bright and glistening yellow of the buttercup may be said never to change. The paler yellows change more easily, but rarely pass to anything but white. Green flowers, not being readily distinguished from the foliage around them, need not be specially mentioned. They are believed to be much rarer than they really are. Horticulturists are able, by cultivation, selection, and hybridization, to cause the colors of flowers to vary in considerable proportions. Not much is known of the laws of these variations, chiefly because gardeners who might tell botanists of them if they would have not the scientific spirit. We cite here what MM. Decaisne and Naudin[7] say respecting the variations of the color of flowers: [Footnote 7: Manuel de l'amateur des jardins.]