For more modern examples the following may be consulted: Blackman and Welsford: _Fertilisation in Lilium_, Annals of Botany, Vol. 27, 1913. Gravis: _Recherches anatomiques sur les organes vegetatifs de l'Urtica Dioica_, Bruxelles, 1885. This memoir contains both good and indifferent plates. Keibel: _Normentafeln zu Entwicklungsgeschichte der Wirbeltiere_, Jena, 1904. Reed: _A Study of the Enzyme-secreting Cells in the Seedlings of Zea Mais and Ph[oe]nix dactylifera_. Annals of Botany, Vol. 18, 1904. Semon: _Zoologische Forschungsreisen in Australien_, Jena, 1904. Vaizey: _On the Morphology of the Sporophyte of Splachnum luteum_, Annals of Botany, Vol. 5, 1890. Woodburn: _Spermatogenesis in Blasia pusilla_, Annals of Botany, Vol. 27, 1913. Several memoirs in the _Fauna und Flora des Golfes von Neapel_ (Berlin) are illustrated by excellent lithographic plates. Many good examples of chromolithography also will be found there. CHROMOLITHOGRAPHY. Lithography is much used for the reproduction of coloured pictures and illustrations, the process being termed chromolithography. The principles involved are the same as for ordinary work, but it is necessary to print from several stones, one for each colour. It is obvious that much skill is required, for the employment of different colours will give a large number of secondary and tertiary tints when printed one above the other in various combinations. Thus, by printing part of a design in yellow and the other part in blue, the finished product would show three colours--yellow, green and blue, and by the use of three primary colours a large number of different tints may be obtained. As already mentioned, each colour is printed by a separate stone, there is thus no limit--excepting that of expense--to the number of different colours which can be obtained. In practice it is usual to make an outline of the essential parts of the composition on a stone, known as the keystone, which is not necessarily used in printing the picture. An impression of this outline is taken upon a sheet of paper, which is used to transfer the design on to the stones, on each of which the artist will draw only those parts which he desires to be printed in one particular pigment. Although the sequence of colours is generally blue, red and yellow, it is obvious that various changes in this order must be made according to the colours used and the exact tint required. For instance, a body colour such as cadmium yellow would precede a glaze such as madder-lake; again, two distinct tints may be obtained from red and blue, for example, according to the order of printing--red upon blue will give a mauve, whilst blue upon red will give a purple. A knowledge of pigments is thus all important, and in printing, the superposition must be perfect.