These six groups were the dominant types throughout the period, but during Upper Carboniferous time three other groups arose, the Coniferales, the Cycadophyta, and the Ginkgoales (of which Ginkgo biloba is the only modern representative).
It is needless to discuss at length the origin of the Gymnosperms. The two views which find most favour in regard to the Coniferales and Cycadophyta are: (I) that both have been derived from remote filicinean ancestors; (2) that the cycads are the descendants of a fern-like stock, while conifers have been evolved from lycopodiaceous ancestors.
The pollen-grains when mature consist of three cells, two small and one large cell; the latter grows into the pollen-tube, as in the Coniferales, and from one of the small cells two large ciliated spermatozoids are eventually produced.
The discovery by the Japanese botanist 'Erase of the development of ciliated spermatozoids in the pollen-tube of Ginkgo, in place of the non-motile male cells of typical conifers, served as a cogent argument in favour of separating the genus from the Coniferales and placing it in a class of its own.
In order to avoid confusion in the use of the term Coniferae, we may adopt as a class-designation the name Coniferales, including both the Coniferae - using the term in a restricted sense - and the Taxaceae.
The most striking characteristic of the majority of the Coniferales is the regular manner of the monopodial branching and the pyramidal shape.
It is in the nature of the secondary xylem that the Coniferales are most readily distinguished from the Dicotyledons and Cycadaceae; the wood is homogeneous in structure, consisting almost entirely of tracheids with circular or polygonal bordered pits on the radial walls, more particularly in the late summer wood.
Araucaria, the leaf-traces persist for a considerable time, perhaps indefinitely, and may be seen in tangential sections of the wood of old stems. The leaf-trace in the Coniferales is simple in its course through the stem, differing in this respect from the double leaf-trace of Ginkgo.
The chief home of the Coniferales is in the northern hemisphere, where certain species occasionally extend into the Arctic circle and penetrate beyond the northern limit of dicotyledon ous trees.
Coniferales: " Report of the Conifer Conference " (1891) Journ.
(1899); Veitch, Manual of the Coniferae (London, 1900); Penhallow, " Anatomy of North American Coniferales," American Naturalist 0904); Engler and Pilger, Das Pflanzenreich, Taxaceae (1903); Seward and Ford, " The Araucarieae, recent and extinct," Phil.
C. Jeffrey, " The Comparative Anatomy and Phylogeny of the Coniferales, part i.