The shoots in Selaginella are characterized by the possession of meristems (dorsal and ventral) at each branching. These angle-meristems produce roots. In contemporary literature these outgrowths are designated as ‘rhizophore’.
However, studies of auxin transport in these structures have conclusively proved their nature as root. Employing labelled auxin (C IAA), it is shown that auxin transport in rhizophores of S. willdenowii is acropetal, whereas it is basipetal in stems. This is comparable to angiosperm roots and stems.
In several species of Selaginella the angle-meristems also develop into leafy shoots. It is particularly true when these angle-meristems are isolated from the influence of parent shoot apex, either through injury or surgical operation. Angle-meristems in stem segments of S. martensii and S. lobbii on isolation from intact plant developed into shoots, and if the cut portions of segments were treated with auxin they developed into roots.
An investigation of regeneration of dorsal as well as ventral angle-meristem has been studied in segments taken from young, middle-aged, and old portions of S. martensii. The ventral meristems, which normally form roots on intact plants in all the three types of excised segments mostly, formed shoots (Fig. 8.5A, B). When the stem segments were smeared with auxin (in lanolin paste) at their distal ends, the ventral meristems differentiated into roots (Fig. 8.5C, D) in all the three types of segments.
The dorsal meristem differentiated as a root (Fig. 8.5C), or a shoot (Fig. 8.5D). These results indicate that auxin controls the development of ventral meristem as a root, in decapitated apices, and it replaces the effect of stem apices, operative in an intact plant.
Differential behaviour of dorsal and ventral angle-meristems in Selaginella has been explained. In S. willdenowii the ventral angle-meristem at the third branch junction behind the shoot tip is the first to become active and grows out as root. The dorsal angle-meristem remains inactive except at the fifth or sixth branch junction from the apex; it develops as a shoot.
In this species, at the branch junctions, twice as much auxin is transported on the dorsal side as on ventral side. Based on this data of auxin transport it has been proposed that the angle-meristems immediately behind the shoot apex are inhibited to grow by high levels of auxin produced by the main apex and branches.
However, some distance away from the apex there is a lower concentration of auxin on ventral side of branch junction because the auxin is transported to the dorsal side.
Low level of auxin permits the growth of angle-meristem on ventral side, and this level determines it to be a root. Further removed from the apex, where lateral branches mature and supply less auxin to the main axis, the growth of dorsal meristem is favoured. At such a low level it differentiates as a shoot.
