The scales upon the Lepidodendron stems, as well as those in the cones, are arranged in a spiral manner, in the same way as plants of the present day. Professor Alexander Dickson has examined the phyllotaxis of Lepidodendrons, and gives the following results of his observations (Trans. Bot. Soc. Edin. xi. 145). The fossil remains of Lepidodendrons are often so compressed that it is difficult, or even impossible, to trace the secondary spirals round the circumference of the stem. In those cases, however, where there is comparatively little compression, i.e. where the stem is more or less cylindrical, the determination of the phyllotaxis is easy. Of such stems he has examined fifteen specimens, which may be classed according to the series of spirals to which the leaf-arrangement belongs:—
A. Ordinary series, ½, ⅓, 2/5, ⅜, 5/13, etc.
(a.) Single spirals (D turning to the right, S to the left).
(1.) Lepidodendron (Possil Ironstone series). Stem about ¾ of an inch in diameter. Secondary spirals 8 D, 13 S, 21 D. Divergence = 13/34 (or possibly 21/55).
(2.) Lepidodendron (Knightswood, near Glasgow, Mr. J. Young). Stem about 1½ inch in diameter. Secondary spirals 13 D, 21 S, 34 D. Divergence = 21/55.
(3.) Lepidodendron (Possil Sandstone series). Trunk about 2 feet long, with an average diameter of 20 inches. Steepest secondary spirals 55 S, 89 D. Divergence = 55/144.
(b.) Conjugate spirals.[11]
(4.) Lepidostrobus ornatus (Bathgate coal-field). About ¾ of an inch in diameter. Secondary spirals 10 D, 16 S, 26 D, 42 S. Divergence = 13/(34×2) (Bijugate arrangement).
(5.) Lepidostrobus (Plean, Stirlingshire, Mr. Mackenzie). About ½ an inch in diameter. Secondary spirals 9 S, 15 D, 24 S, 39 D. Divergence = 8/(21×3) (Trijugate arrangement).
(6.) Knorria taxina (from collection of Dr. Rankin, Carluke). Somewhat compressed, 2-2½ inches[12] in diameter. Secondary spirals 15 D, 24 S. Divergence = 15/(13×3) (Trijugate arrangement).