The transition from vegetative to reproductive growth of shoot apices of holoheterophyadic species of Equisetum: phenology, morphology, and anatomy

Abstract

Rhizomes of Equisetum arvense and E. telmateia were excavated to determine the time course of strobiliferous bud development. Cone initiation occurs during July, August, and September. The vegetative apex has a single, pyramidal apical initial cell which produces derivatives (merophytes) that by further division form the nodes and internodes. At transition of the apex to the reproductive state, the apical initial and its most recent derivatives cease division activity. Cells lower on the flanks of the apical dome appear to increase mitotic activity, resulting in an increase in the size of the apical dome and in the number of flank cells and a change in form. The vegetative nodal sheath plastochron becomes interrupted at the initiation of a whorl of leaves that does not grow upward but forms the annulus. After the dome enlarges, rapid initiation of successive whorls of sporophylls occurs. These differ from leaves in being mound shaped, rather than upwardly pointed. Thus Equisetum is like the seed plants in that the transition of its shoot apex from vegetative to reproductive growth is accompanied by increased division activity, increased size, change in zonation, and change in plastochron.