Studies on Vein Formation in the Leaf of the Fern Thelypteris palustris Schott

Abstract

Sporophytes of the fern Thelypteris palustris Schott. were cultured from gametophytes, and leaf venation was examined in three ways. First, the vein pattern was evaluated over the heteroblastic sequence as venation shifts from dichotomous through anistomous to pinnate branching. The ratio of total vein length to leaf area is constant for leaves over this heteroblastic sequence, and pinnate branching alternates between left and right with an occasional pair of consecutive branches on the same side. Second, a new method of study, tracheid analysis, evaluates the length and deployment of tracheids in leaves as well as those of general epidermal cells. A terminal vein cell enlists a new cell into a vein from a hemispheric area of ca. 20 cells; vein branching occurs by two independent events of cell enlistment, one for each of two new branches. Vein fusion is not true fusion but an anastomosis where two veins run parallel along a leaf region. Third, puncturing experiments were performed on leaves in the fiddlehead stage. A damaged vein terminus leads to cessation of both vein extension and general growth in the injured region, whereas damage to marginal and intercostal areas does not alter development. These studies indicate that (a) a vein lengthens by enlistment of an unspecialized cell to become a specialized terminal cell of the vein; (b) this newly enlisted cell is one in a hemispheric field of ca. 20 uncommitted cells; and (c) timing of vein branching is determined more by the size of this field of cells rather than by the status of the vein itself. These features are compatible with Sachs's hypothesis of preferred channeling rather than Meinhardt's Turing type of model or Goebel's postulate that procambial cells are derived directly from the marginal meristem.