Root of Pteridophyte

  Structures of Root of Pteridophyte

The earliest diverging branched axes of extant vascular plants, lycophytes, are characterized by the presence of purely structural roots whose only function is anchorage, and these plant groups include forms with very complex rooting systems. The rooting structures of the most basal branch is a single unbranched rooting axis strongly resembling a root-like organizational strategy of geotropically-growing axes found in the common ancestor of mosses and liverworts. Aquatic rooted forms of this grade of morphological diversity became established in early prediluvial times when no earth surface was free from water. Structures on axes that carried out rooting functions in plants appeared very early in the evolutionary history of land plants. The earliest land plants did not have true roots as they are defined today, that is, as organs carrying out anchorage and absorption functions that developed from a root meristem covered with a root cap. Instead, primitive rooting structures were unicellular or multicellular, non-branching and non-tip-growing structures called rhizoids which carried out anchorage and absorption in both early non-vascular and vascular land plants.

The rooting organ of the earliest land plants was simple, consisting of axes with tiny unbranched hair-like projections. Rhizoids of mosses are highly diverse structurally and they can be both multicellular and unicellular. Unicellular and multicellular rhizoids of non-vascular bryophytes carry out rooting functions solely in their free-living gametophytes. 

Given their limited proteoid function, it is widely accepted that rhizoids of this type only arose after the acquisition of stomata. In the model angiosperm, A. thaliana, rhizoidal multicellular epidermal structures called cap cells differentiate from protodermal cells in the post-transition zone epidermis of the transition zone while in the lycopsid, S. moellendorffii, a cell-fate determinant, SCL4, restricts the pre-pattern at an abaxial flank of the apex. This work shows that, despite differences in broad morphological form, the molecular and genetic mechanisms underlying the formation of both rhizoids and roots share considerable conservation. Field studies show that rooting structures in tree lycopods were dicots, indicating a shared origin and history with roots in their vascularly-formed spores