Origin of the Coleorhiza in Cycad Seedlings and its Structural Homology with That of the Poaceae

Abstract

In the literature there is disagreement about the existence of a coleorhiza in cycad embryos. In this paper the terminology of the cycad ovule, seed and embryo is revised. It was confirmed that the cycad ovule and seed are pachychalazal and that the seed coat is exclusively formed by the pachychalaza. The term 'pleurotesta' as a substitute for the so-called 'endotesta' is suggested to describe the inner, membranous part of the seed coat. The anatomy of the cycad embryo was studied in comparison with the grass embryo and it was found that a coleorhiza does exist in cycad embryos and derives from the distal part of the suspensor. It is postulated that the coleorhiza in grasses also derives from the distal part of the suspensor and that the two structures are therefore structurally homologous.

Coleorhiza‐enforced seed dormancy: a novel mechanism to control germination in grasses

“…Although the micropylar endosperm and coleorhiza share similar anatomical compartments, as organs surrounding the radicle in both monocot and eudicot diaspores, the micropylar endosperm and coleorhiza have distinct developmental origins. The micropylar endosperm is a triploid endospermic tissue formed from the double fertilisation of polar nuclei (Sreenivasulu & Wobus, 2013; Yan et al ., 2014), whereas the coleorhiza is proposed to be homologous to the hypocotyl or suspensor (Tillich, 2007; Robbertse et al ., 2011). The coleorhiza is widespread beyond the grasses in other monocot species and in cycads but is absent in eudicots.…”

“…Grass embryos lack the micropylar endosperm that plays an important role in eudicot seed dormancy and germination. Instead, the coleorhiza, a nonvascularised embryonic organ that expands upon imbibition, emerges from the caryopsis and is thought to have some role in protecting the growing embryo (Tillich, 2007; Robbertse et al ., 2011). While the coleorhiza is widespread beyond the grasses in other monocot species and in cycads, it is absent in eudicots.…”