Page:Journal of botany, British and foreign, Volume 34 (1896).djvu/349

 FOSSIL PLANTS OF THE COAL-MEASURES. 323 and habit of the plant. To quote Williamson and Scott's words — " We must, therefore, picture Lyginodendron to ourselves as having a tall, upright stem rising to a height of several feet, and bearing somewhat remote, spirally arranged, highly compound fern-leaves. The base of the stem, where the adventitious roots were given oft', must have been buried for some depth in the earth or mud." The centre of the stem was occupied by a parenchymatous pith con- taining characteristic "nests" of sclerous cells. At the periphery of this medullary tissue, occurs a ring of detached strands of primary xylem, each consisting of spiral, scalariform and pitted trachea with a small amount of parenchyma. External to these primary groups is a ring of secondary wood, made up of radially arranged pitted tracheae and medullary rays ; the phloem consists of parenchyma and sieve-tube-like elements, and, like the xylem, is traversed by medullary ray cells. In some specimens patches of primary phloem have been recognized external to the secondary phloem, and between the secondary xylem and phloem a well- marked cambium is shown in wonderful perfection in some of the drawings accompanying the paper. Passing beyond the vascular tissue, we find a pericycle which at an early stage in the growth of the plant gave rise to a periderm. The cortex proper consists internally of parenchymatous tissue containing nests of sclerous cells ; external to this, alternating radial bands of parenchyma and sclerenchyma constitute the exceedingly characteristic outer cortex of the stem. Secretory sacs are abundant in the pith, medullary rays, and inner cortex. Leaf-trace bundles are constantly seen in transverse sections on their way out to the petioles. Those met with in the secondary xylem, are in the form of single groups of primary xylem with a fan-shaped group of secondary tracheae on the external face of the primary strand. If such a leaf- trace bundle be followed to the petiole, it is found to pass through five internodes before entering the pericycle ; during this part of its course it loses its secondary xylem, and begins to divide into two bundles. Finally the twin bundles pass gradually outwards until they enter the petiole. The primary xylem strands of the stem have been shown to be the downward prolongations of the leaf-traces ; hence the bundle- system of Lyginodendron is essentially a leaf- trace system. The most important points with regard to the detailed structure of the primary bundles are (1), their collateral nature, and (2), the occurrence of the protoxylem in an internal position in each primary xylem -strand. In short, the structure of each primary bundle agrees very closely with that of the foliar bundles in recent Cycads. The secondary wood also exhibits a striking resemblance to the secondary wood of cycadean stems. The petioles of this genus are of the type previously known as liachiopteris aspera Will,, and the pinnules are clearly such as conform to the characters of a typical Sphenopieris. It is a fact of importance that the collateral leaf-trace in the stem assumes a concentric structure as it passes into the petiole. Finally, it is now perfectly clear that Williamson's Kaloxylon Hookeri is the adventitious root of Lyginodendron,