Fig. 152. Lepidodendron vasculare. Sections of dichotomously branched shoot.
- From a section (10·5 × 9 cm.) in the Cambridge Botany School Collection.
- From a section (8 cm. long) in the Cambridge Collection.
The section represented diagrammatically in [fig. 152], A, has lost the outermost part of the cortex together with the leaf-cushions; it consists largely of secondary cortex composed of radially disposed phelloderm cells and tangentially placed secretory strands (represented by the discontinuous black lines in the drawing): the dotted region in the central part of the axis is composed of primary cortical parenchyma, and the two spaces surrounding the steles contain portions of the lacunar middle cortex. Each stele possesses a narrow crescentic zone of secondary xylem; the amount is greater in the case of the right-hand stele, of which a small piece is shown on a larger scale; the striking contrast in size between the outer and more internal secondary tracheae is no doubt the expression of some unfavourable condition of growth. The position of the secretory zone beyond the secondary xylem is shown at sc, [fig. 152], A.
Fig. 153. Lepidodendron vasculare.
(From a specimen (16 × 7·5 cm.) in the Manchester Museum.)
An example of a large and partially decorticated stem is afforded by the specimen (16 × 7·5 cm.) shown in [fig. 153]. The irregularly ribbed surface is formed of rather thick-walled phelloderm, in which occur tangentially arranged rows of secretory strands. The tapered form of the secondary cortex as it abuts internally on the primary cortex is shown very clearly in the drawing (cf. [fig. 151], C). The stele in this much older stem consists mainly of secondary wood.
Fig. 154. Lepidodendron vasculare. Shoot (2·8 cm. diam.) with two steles. (From a specimen from Halifax in the Williamson Collection, British Museum, No. 340.)
An interesting example of a small shoot, the largest diameter of which is 2·8 cm., is shown in [fig. 154], A: the section was cut a short distance above the bifurcation of the stele into two approximately equal branches. The outer part of the cortex consists of phelloderm, pd, with the usual rows of secretory tracts, and primary outer cortex g; the middle cortex is represented by patches of parenchyma with a few leaf-traces. To one of the steles, s′ ([fig. 154], A), a crescent-shaped band of secondary xylem has been added; the other stele, S, possesses no fully developed secondary elements.