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Floral Nectar Production and Nectary Anatomy and Ultrastructure of Echinacea purpurea (Asteraceae)
by Wist T. J., Davis A. R. (2006)
Department of Biology, 112 Science Place, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2
in Ann Bot. 2006 Feb; 97(2): 177–193. – doi: 10.1093/aob/mcj027 –
• Key Results Florets were protandrous with nectar being secreted from anthesis until the third day of the pistillate phase. Nectar production per floret peaked on the first day of stigma receptivity, making the two innermost whorls of open florets most attractive to foraging visitors. Modified stomata were situated along the apical rim of the collar-like nectary, which surrounds the style base and sits on top of the inferior ovary. The floral nectary was supplied by phloem only, and both sieve elements and companion cells were found adjacent to the epidermis; the latter participated in the origin of some of the precursor cells that yielded these specialized cells of phloem. Companion cells possessed wall ingrowths (transfer cells). Lobed nuclei were a key feature of secretory parenchyma cells.
This single cell layer was composed of modified stomata dispersed among non-specialized epidermal cells. Trichomes, and subsidiary cells around the paired guard cells per stomate, were absent.
On average, 29·2 modified stomata were present on the nectary surface, and the data in Table 1 suggest that additional modified stomata are not initiated after the mature bud phase is attained. The modified stomata were densest along the uneven upper rim of the nectary (Figs 2A and B and 4A and B), although they were oriented in different planes and were rarely adjacent to one another (Figs 2B and 4A and D). Less frequently, stomata were located below the nectary rim, positioned on both the exterior and interior nectary surfaces (Figs 2B, bottom right, and and4E).4E). Along the rim, the two kidney-shaped guard cells of each modified stomate were slightly raised (Fig. 2B and D–F), but were not raised in those stomata located on the external and internal walls of the nectary (Figs 4D and E). Each guard cell possessed a centrally located nucleus (Fig. 4C, top left, and D, top right) and abundant amyloplasts (Fig. 4A–F).