Secondary Wall Formation in Elaters of Liverworts and the Hornwort Megaceros

Abstract

Elaters of most liverworts and some hornworts are characterized by prominent spiral secondary walls. Investigation of elater development in two liverworts, Symphyogyna and Radula, and the hornwort Megaceros, showed similarities in the process of secondary wall formation but significant differences in secondary wall structure. Elaters elongate before secondary wall deposition and have a transverse microtubule array indicative of diffuse growth. After elongation, microtubules aggregate into spiral bands that predict the pattern of the future secondary wall: two narrow spirals in Symphyogyna and Radula and a single broad spiral in Megaceros. Secondary walls of liverwort elaters are composed of a single electron‐dense layer, but secondary walls of Megaceros are formed of three distinctive layers: an outer electron dense layer, a central fibrillar layer, and a thin inner layer. Elaters of liverworts and Megaceros stain with TUNEL, which indicates that they may undergo some aspects of programmed cell degeneration after secondary wall development.