D.W. Stevenson
Dioon Lindl. Edward’s Bot. Reg. 29: misc. 59-60 (1843); from the Greek dis two and oon eggs, in reference to the paired seeds. Type: Dioon edule Lindl.
A genus of 11 described species with several unresolved complexes, in the family Zamiaceae; Mexico, Honduras & Nicaragua Distinguishing characters:
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Genus description: Habit: dioecious palmlike shrubs with aerial or subterranean, pachycaul, cylindrical stems, with usually many leaves. Leaf bases persistent or shedding to leave smooth bark in different species. Basal offsets ("suckers") produced by most species, aerial branching uncommon. Leaves: pinnate, spirally arranged, interspersed with cataphylls, lower leaflets often reduced to spines. Petioles lacking prickles, usually swollen at the base. Longitudinal ptyxis erect, horizontal ptyxis erect. Leaflets simple, frequently with spiny margins, with numerous bifurcating parallel veins and no distinct midrib, leaflets broad-based, not articulated, inserted near the edges of the rhachis towards the adaxial side, lacking a differently coloured basal gland; stomata on lower surface only or on both surfaces; epidermal cells elongated parallel to long axes of leaflets. Leaves pubescent, at least when young, with branched or simple transparent hairs. Microsporophylls: spirally aggregated into determinate, stalked male cones and each with a simple sterile apex, which is flattened and upturned. Each microsporophyll bearing numerous microsporangia (pollensacs) on its abaxial surfaces. Microsporangia opening by slits. Pollen cymbiform, monosulcate. Megasporophylls: spirally aggregated into determinate, stalked female cones. Sporophylls with a simple dilated apex which is flattened and upturned. Ovules two (rarely three), stalked, orthotropous, inserted on the inner (axis-facing) surface of the thickened lamina and directed inwards ("inverted"). Seeds: subglobular to oblong or ellipsoidal, with a white or cream fleshy outer sarcotesta. Endosperm haploid, derived from the female gametophyte. Embryo straight; with 2 cotyledons that are usually united at the tips and a very long, spirally twisted suspensor. Seeds radiospermic; germination cryptocotular. IDENTIFICATION KEYS Try this experimental multiple-entry key based on the DELTA format. There is also a more traditional dichotomous key lower on the page. To use this key, start with one or two characters that are clear. To refine a query, use the BACK button of your browser, return to the original query, and gradually add more characters. Dichotomous key
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