Function and Development of the Primary Thickening Meristem in the Monocotyledon, Allium cepa L.

The small stem of the common onion, Allium cepa, possesses a primary thickening meristem (PTM) and shows extensive primary thickening, or growth in width, during development. The growth of the vegetative and reproductive axes was observed through the biennial life cycle. Tangentially flattened cells, diagnostic of the PTM, are seen 6 wk after sowing of seeds, but localized cell divisions in the region of the stem in which the PTM will develop appear as early as 10 days after sowing. The PTM is present throughout vegetative growth, differentiates into the axillary buds, but does not differentiate into the inflorescence axis. The PTM, therefore, is associated with growth in width of the vegetative axis but not growth in length of the floral axis. The PTM also gives rise to shoot-borne roots. Primary thickening is accomplished by (1) cell divisions and cell enlargement in cell files oriented vertically in the stem near the apex and (2) subsequent reorientation of the established cell files to a horizontal angle.