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 botany, the subject with which his name is now most intimately associated, as it occupied all the latter part of his career as an investigator. His interest in such matters goes back, as has been mentioned above, to the very beginning of his scientific life. In addition to his work for Lindley and Hutton, a paper of his on the origin of coal was read before the British Association as early as 1842. His first original contribution to fossil botany dates from the year 1851, when he published a paper "On the Structure and Affinities of the Plants hitherto known as Sternbergiæ," in which he demonstrated their true nature as casts of the pith-cavity of Gymnospermous trees. A few years later, in 1854-5, he published papers on what was then called Zamia gigas, an extraordinary oolitic fossil, which Williamson believed to have Cycadean affinities, a view which has since been so far confirmed that the fossil is now regarded as representing the fructification of one the Bennettitee, an allied, though very different family. Williamson's full memoir on the subject was written soon after 1855, but, owing to a succession of misfortunes, its appearance was long delayed, and it only saw the light in the Linnean Society's Transactions' for 1868, when it was published simultaneously with Mr. Carruthers' well-known paper on fossil Cycadean stems. The latter author founded a new genus for Zamia gigas under the name of Williamsomia.

Williamson's really characteristic work in fossil botany consisted in the investigation of the histological structure of carboniferous plants. The first beginning was made with the paper on Sternbergia, but it was not till long afterwards that the long series of publications began, which have done more than the works of any other writer to make us acquainted with the organisation of Palæozoic plants. It was early in the fifties that Williamson made his first sections, but not till 1868 that, in consequence of a correspondence with the French paleobotanist, Grand'Eury, he published the result of his investigations in the paper "On the Structure of the Woody Zone of an undescribed Form of Calamite," 'Manchester Literary and Philosophica Society's Proceedings,' Ser. 3, vol. 4. From that period onwards, his whole time available for original research was devoted to the Carboniferous Flora, and a magnificent series of memoirs was the result, which will always rank among the classics of fossil botany. The Royal Society alone published in the Philosophical Transactions' nineteen memoirs from his hand, their dates ranging from 1871 to 1893, and, besides these, many valuable papers appeared elsewhere, notably the memoir on Stigmaria ficoides, published in 1886, by the Palæontographica! Society. It is impossible here to attempt anything like a summary of this great work, which threw light on every department of Palæozoic botany.