THE origin of the conifers is still a matter of uncertainty. It is commonly supposed that they were derived from the same stock as Cordaites, but, though they certainly existed in the Upper Coal Measures, they only became abundant during the Mesozoic era. All the present-day types are well represented : pines, yews, Araucarias and others. In addition there are forms which seem to combine the characters of more than one group, cither in the structure of their wood or of their cones, and numerous others which belong to extinct families. Two instances of changed geographical distribution may be mentioned : the Araucarias (which include the Norfolk Island pine and the Monkey-puzzle) are now confined to the southern hemisphere, but were formerly abundant all over the world; and the Sequoias, which are known from the latter part of the Mesozoic and were widely spread over the northern hemisphere almost throughout the succeeding Tertiary era, are now reduced to two species (the Mammoth-tree and the Redwood) growing only in a narrow belt of country in California, where they are remarkable for their size and longevity.
The Maidenhair-tree (Gingko) is now unknown in the wild state, and probably owes its survival to its cultivation in the gardens of Eastern temples. It is the sole living representative of a group which in the Mesozoic era had an almost world-wide distribution and showed a very considerable range in habit and structure. The, which are more frequent as fossils than the other organs, may be entire or slightly notched and fan-shaped, like those of the cultivated Maidenhair-tree, or long and deeply divided, often with very narrow linear segments. They always have characteristic forking veins, and the microscopic structure of the breathing-pores is always of the same general plan. Like most leaves
of gymnosperms, recent and fossil, they were tough and resistant, and are often preserved in Mesozoic rocks in a mummified state, flattened between the layers of sediment, but with the outer cuticle practically unchanged. Such specimens, after being removed from the rock and cleared by appropriate chemical treatment, can be examined microscopically; the differences in structural detail so revealed, especially of the breathing-pores (stomata), have proved to be valuable guides to affinity. The conifers and the cycado-phytes (which we are about to discuss) have also been intensively studied in this way.