In this article we will discuss about the anatomy of psilotum with the help of a suitable diagram.
Growth of rhizome and aerial branches takes place by divisions of a single apical cell. During dichotomy it divides equally, forming two apical cells which result in two meristems.
Internal structure of rhizome varies with its diameter. The slender rhizome (about 1 mm in diameter) is a simple parenchymatous structure. An increase in diameter is accompanied with cellular differentiation. To begin with, in the centre can be seen elongate cells. Larger rhizome has a well-developed central vascular cylinder (Fig. 7.1G) which is a typical protostele (central core of xylem surrounded by phloem).
The xylem lacks differentiation into protoxylem and metaxylem. Structure of stele also varies with the diameter of rhizome. Smaller rhizome has a compact vascular cylinder but in larger rhizome the xylem mass is dissected to various degrees by differentiation of parenchyma. Cortical cells of rhizome are infested with fungus. In addition to its avascular to vascular nature which depends on its thickness, in the rhizome can be seen longitudinal interruptions of stele.
In aerial axis also, the vascular cylinder varies all along its length. The basal part is similar to rhizome, in having a protostele which is with or without stelar parenchyma and ill-defined pith. Higher up, the vascular cylinder is siphonostelic with sclerenchymatous pith. The xylem is sharply angled (stellate); pentarch to octarch in the main axis (Fig. 7.1H) and triarch to diarch in distal region (Fig. 7.1I).
Protoxylem points are at the ends of stellate xylem (exarch) and poorly developed phloem surrounds xylem. A light microscope has failed to provide detailed information about structure and development of phloem as compared to electron microscope.
In the cortex, three or four zones (Fig. 7.1H) can be identified. The innermost zone is marked with its brown cell contents phlobaphene, an oxidation condensation product of tannin. It is followed by a parenchymatous zone, without intercellular spaces, and the third zone is sclerenchymatous. The last cortical zone comprises vertically elongated cells which are full of chloroplasts.
As the plants lack leaves this cortical zone is assimilatory in function. Small intercellular spaces in this region connect to the outer atmosphere through stomata in the cutinized epidermis. The stomata are slightly sunken, confined to furrows and are with small substomatal chambers. The stomata are without special subsidiary cells. It is comparable to the situation in gymnosperms.
Internally, an appendage consists of photosynthetic parenchyma cells. There is no vascular bundle in the appendage of P. nudum but in P. complanatum, a leaf trace ends at the base of the foliar structure.