The rhizome in Pteridium is subterranean with numerous dichotomous forkings and is capable of indefinite growth. Pteridium aquilinum, the common bracken fern, is the first to invade cultivated fields and when once established is difficult to eradicate. This is one of the ferns able to compete with flowering plants as a weed.
It is mainly due to the great depth at which its rhizome can penetrate the soil. Slender adventitious roots occur on the rhizome and on its upper surface arise leaves (Fig. 10.1 A). The leaf lamina is trilobed, and tripinnate at its lower portion.
The sporangia form continuous sori along the margin of pinnules (Fig. 10.1B) and are surrounded by two-lipped indusium (Fig. 10.1C). The upper lip is relatively thick but the lower one is thin and papery. The sorus is marginal in origin and the outer and inner indusial flaps arise laterally on the young receptacle. The sori are of mixed type and sporangia are leptosporangiate.
The annulus is a vertical row of cells encircling about three-fourth of the sporangium. Mature cells of annulus are thickened along their radial and inner tangential walls. Immediately below the free end of the annulus is a vertical strip of four cells, the stomium. The sporangia produce 64 spores and the gametophytes are heart-shaped.
First-formed portion of stem is siphonostelic but later-formed portion has dicyclic to polycyclic stele. Pteridium aquilinum has a dicyclic central cylinder (Fig. 10.1D). The outer cylinder is a much dissected dictyostele and the inner one consists of only two strips. Each meristele has an endodermis, pericycle zone of one or two cells and followed by it are phloem and xylem.
On the periphery the phloem elements comprise small sieve tubes, the protophloem, and larger phloem parenchyma cells. The inner region of phloem is metaphloem composed of large sieve tubes. At the centre are one or two strands of proto-xylem. External to protoxylem and separated from it by parenchyma cells is metaxylem consisting of scalariform tracheids.
True vessels have been recorded in Pteridium. The central portion of stem is parenchymatous but in between the two cycles of vascular cylinder is sclerenchyma, either completely encircling the inner cyclinder or form two strips.
The outer region of cortex is sclerenchymatous but is interrupted on either side by a zone of parenchyma. The parenchymatous region, interrupting sclerenchyma, has been interpreted as aeration tissue or pneumatophore.
Another common member of Pteridaceae is Pteris having a continuous sorus on margin, but it is superficial in origin and margin becomes inrolled to protect it (Fig. 10.1E). Some species of Pteris have somewhat expanded soral region. It is indicative of a step in the evolution (acrostichoid condition).
