Another large group of casual abnormalities is concerned with the colour of leaves. The Purple Beech is a case in point. It was not produced by selection, but arose naturally, no doubt as a chance seedling. In this instance the character is usually passed on to the offspring, most seedlings having similar purple leaves, though some individuals are green. The peculiar colour is due in this case to a pigment in the epidermis of the leaf; the green chlorophyll is duly present, though its colour is masked by the purple leaf-skin. To a different category belong the “gold” and “silver” variegations which are so much exploited in shrubberies and borders and greenhouses. These spots or stripes or tintings of pale colour on the leaves are due to the lack of chlorophyll in the chromatophores (chlorophyll corpuscles); sometimes to an absence of the chromatophores themselves; and this omission appears to be caused by an enfeebled condition of the plant. Variegated plants are weaker than normal ones, and hence do not tend to survive in nature. But gardeners have protected and propagated a large number of them. When the variegation arises, as it often does, on a branch of an otherwise normal plant, it usually is not reproducible from seed, and must be perpetuated by cuttings. But where it happens with seedlings, it is often more or less fixed, and may be reproduced generation after generation, as in the Golden Elder, Golder Feather, and the marginal-variegated form of Winter Cress (_Barbarea vulgaris_).