The Cycad Pages
GENUS  Dioon
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.
Platyzamia Zucc., Abh. Math.-Physik. Cl. Konigl. Bayer. Akad. Wiss. 4: 23, t. 4 (1845). Type: Platyzamia rigida Zucc.

A genus of 11 described species with several unresolved complexes, in the family Zamiaceae; Mexico, Honduras & Nicaragua
Distinguishing characters:

  • Leaves pinnate
  • Leaflets lacking a midrib
  • Leaflets not articulated
  • Sporophylls not in vertical rows in cones
  • Megasporophyll apices broadly flattened, upturned, overlapping

Select a species, or go to the keys to species.
Individual species pages will have literature references and synonyms, and eventually images (indicated by I if present), distribution maps (M), descriptions (D) and general notes (N) on the species. These are not yet all in place for all species, but the database is slowly growing. On species pages, click on thumbnail images to see full-screen images. links to images of type specimens held by the New York Botanical Garden (NY). links to images of the protologue or original description pertaining to the name.

D. califanoi M     N D. holmgrenii M     N D. purpusii M     N D. spinulosum M I   N
D. caputoi M   N D. mejiae M   N D. rzedowskii M   N D. tomasellii M   N
D. edule M   N D. merolae M   N D. sonorense M   N

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.

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.
Leaf length (cm)
Leaf - keel
Basal leaflets
Leaflet - shape
Leaflet-to-rachis angle
Leaflet - margins
Leaflet - margins
Leaflet - width (cm)

Dichotomous key
1 Leaflets falcate, especially nearer to the petiole
2 Leaflets more than 5 mm wide -- Dioon tomasellii
2* Leaflets less than 5 mm wide -- Dioon sonorense
1* Leaflets straight
3 Leaflets less than 15 mm wide
4 Leaflets with entire margins -- Dioon edule
4* Leaflets with spinescent margins
5 Leaflets at ca 90 deg to rachis
6 Leaflets 10-12 mm wide, overlapping -- Dioon merolae
6* Leaflets 7-9 mm wide, not overlapping -- Dioon holmgrenii
5* Leaflets at 45 deg or less to rachis
7 Leaflets less than 5 mm wide, not overlapping; leaves flat -- Dioon caputoi
7* Leaflets more than 5 mm wide, overlapping; leaves keeled
8 Leaflets 7-8 mm wide -- Dioon califanoi
8* Leaflets 8-10 mm wide -- Dioon purpusii
3* Leaflets more than 15 mm wide
9 Petiole spine-free -- Dioon rzedowskii
9* Petiole spinescent
10 Leaflet margins entire -- Dioon mejiae
10* Leaflet margins spinescent -- Dioon spinulosum

The Cycad Pages

© 1998-2012 Royal Botanic Gardens Sydney
Written and maintained by Ken Hill 1998-2010
Maintained by Leonie Stanberg and Dennis Stevenson 2010-2012
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