In this article we will discuss about the anatomy of selaginella with the help of a suitable diagram.
Apical growth takes place by a single cell and its derivatives or by an apical meristem, comprising a group of cells. Branching occurs by a bifurcation of shoot apex into two apices, one of which overtops the other resulting in anistomous branching. However, new branching in S. speciosa occurs in lateral position on the dome-shaped apical meristem.
Both the main axis and lateral branch are shown to grow. Leaves originate from superficial cells, located around the apical meristem. In a study of origin and development of leaves in S. martensii it was found that each pair of leaves, dorsal and ventral, originate about the same time from the meristem.
The ventral leaf primordium is, however, longer from the very beginning than dorsal primordium. Histogenesis is identical in both types of leaves but there is precocious tissue maturation in smaller dorsal leaf.
In a cross section, the leaf has two epidermal layers made up of chlorophyllous cells. Epidermal cells of leaves of isophyllous species are relatively linear and narrower than of heterophyllous species which have polygonal and shorter cells.
The lower epidermis has stomata restricted in the midrib region. Ground tissue is made up of photosynthetic parenchyma; mesophyll with intercellular spaces. A vein runs along the length of leaf. As an exception to this, in two species (S. adunca and S. schaffneri) branched venation system similar to megaphyll has been described.
Stem epidermis is made up of thick-walled cells with or without stomata. The cortex comprises angular cells without intercellular spaces. In larger stems the outer cell layers of cortex get scelerified.
The stele is set off from the cortex (Fig. 8.2B) by a few radially elongated endodermal cells with casparian strips, the trabeculae, a characteristic of the genus Selaginella. Young plants are invariably monostelic. Adult forms are monostelic (S.flabellata), bistelic (Fig. 8.2B) or polystelic, with a wide range of 2-16 separate steles. S. kraussiana is bistelic; the two steals run side by side and are interconnected at the nodes (Fig. 8.2C).
In S. braunii the prostrate axis is bistelic wheres erect axes are monostelic and in S. loyalli prostrate axis is bistelic and erect axes are polystelic, and are 2-16 in number. Larger stems of heterophyllous species have steles as vascular plates which are often associated with small terete meristeles (S. ovidangula).
Stele in Selaginella may be circular (Fig. 8.2D) or ribbon-shaped depending upon the species, protostelic or siphonostelic with exarch protoxylem, and is bound by one-cell thick pericycle, and the endodermal cells form the trabeculae. Xylem is surrounded by two or three layers of parenchyma outside which is a single layer of sieve tubes, all around except the region radial to the protoxylem.
In some isophyllous species (S. rupestris and S. oregana) true vessels occur. Large bundles in stems of Selaginella reveal both mesarchy and exarchy, depending on the level within the stem and the particular pole of maturation observed. Vessels are also recorded in leaves of heterophyllous species such as S. arbuscula.
The primary root is short-lived and roots in the genus are adventitious. In most of the species delicate and sparingly branched structures which develop at the distal ends of ‘rhizophores’ are described as roots. These are typically monarch (Fig. 8.2E) with one phloem and xylem mass. In S. wallichi roots may develop anywhere along the stem.
Root or Rhizophore:
Morphological nature of ‘rhizophore’ has been controversial.
It has been held to be:
(a) Root;
(b) Branch of stem; and
(c) Structure sui generis (falling in none of the categories).
Earlier investigators reported a unique combination of characters of these forms:
(i) Exogenous origin from the stem at the time of branching;
(ii) Lack of root cap;
(iii) Production of roots endogenously behind the tip, and
(iv) Ability in some instances to be converted into a leafy shoot.
Since these features are not typical for a root and these outgrowths were described to bear roots endogenously, they were called ‘rhizophores’.
The features suggestive of its root nature are:
(i) Positive geotropism, and
(ii) Anatomical organization; monarch xylem (even species with polystelic stem have monarch xylem).
The concept of rhizophore in Selaginella has been questioned. Evidence has been presented for its root nature, it is in addition to that of internal organization, recognized by earlier workers. In S. densa, S. wallacei and S. kraussiana when the structures are very short (less than one millimetre) the root cap develops and in S. martensii cap differentiates when it nears the soil.
Moreover, in no instance evidence was found of its endogenous initiation, as described by earlier workers. Studies of auxin transport in these structures also support their nature as root. Using labelled auxin (C14IAA) it has been shown that auxin transport in ‘rhizophores’ of Selaginella is acropetal whereas it is basipetal in stems.
It is comparable to angiosperm root and stem. On a nutrient medium supplemented with auxin the segments of rhizophore continue to grow as root. However, in the presence of TIBA (an antiauxin) some of the segments develop as shoots.
Therefore, the term ‘rhizophore’ as well as the arguments should now be of historical importance. Nevertheless, the rhizophore has been a unique descriptive feature of Selaginella and the term has been so deeply entrenched in botanical literature that it will be difficult to displace.
This continued debate about rhizophore has been given a new lease by results of electrophoretic pattern of polypeptides from stem, root and rhizophore. The study has shown that polypeptides of rhizophore resemble more with that of stem rather than root.