Dioon

Leaf of Dioon spinulosum with are identified by the veining and prickly margin

The leaves are paired pinnate and are spirally on the stem axis. Some leaves are initially wrapped in protective leaves during their development, which are called cataphyll. Unlike other cycads such as Stangeria eriopus, the juvenile fins are not curled either transversely or longitudinally, but straight. The lower leaflets are often reduced to their petiole. The petioles are uncorned and thickened at the base.

The leaflets are simple, often with a prickly edge. The primary nerve consists of many forked dividing nerves, without a recognizable midrib. The nerves spring directly from the edge of the leaf rhachis and then run towards the plumage axis. The spars are hairy at least on young leaves. The hairs (trichomes) are colorless, branched or simple.

The stomata for the gas exchange are found either only on the underside of the leaf, or in some species on both sides. Glands are not otherwise colored and difficult to identify. The cells of the leaf epidermis are extended parallel to the plumage axis.

Like other cycads, coral-like roots are sometimes formed that grow up from the primary root and branch out quickly just below the soil surface. The individual roots are thickened rhizomes and are inhabited by cyanobacteria of the genus Nostoc, which mainly used them for nitrogen fixation. There also seems to be a symbiotic relationship with arbuscular mycorrhizal fungi.[9]

All Dioons are dioecious, having either male or female organs, but only on different individuals. The male cone is stalked. The male cone scales called microsporophyll, which are flattened and bent upwards, sterile at the tip . They are arranged spirally around the pin axis. Each microsporophyll carries on its underside (abaxial side) a variety of pollen sacs. These open with slots, from which the pollen then escapes. The pollen grains are streamlined and have only one germ line (monosulcate). The male cones fall off after one year.

The female cones are also stalked and can remain on the plant for more than a year. The female microsporophyll are more leaf-like than those of other cycads, the greatest similarity to those of the genus Cycas. The scales are flattened at the top, widened and bent up. Each sporophyll carries two, rarely three, ovules. These are straight to the axis (orthotropic), are inclined inwardly to this (inverse) and hang on the axis-facing side of the thickened scales. The opening at the tip of the ovules (micropyle) is very small, leaving only a narrow opening on the Nucellus. The megaspore sheath thickened from 3–4.5 microns in the young ovule to 9-10 microns in the mature seed. The number of archegonia varies between one and ten. The nucleus of the egg is unusually large. The largest female cones of the genus D. spinolosum shows up to 80 centimeters in length and a diameter of up to 30 centimeters.

The seeds are spherical, ovate or ellipsoidal and reach a diameter between about one and four centimeters. They are surrounded by a fleshy, white or cream-colored seed coat. The seed coat consists of three layers, a fleshy outer and inner layer, between which a layer of stone cells lies. The embryo is straight with two cotyledons, these are fused at the top. The embryo carrier (suspensor) is very long and spirally twisted. The seeds are radiated (radiosperm). The germination takes place cryptocotylar, i.e. the cotyledons remain during germination in the seed.

• 
  Female cone of Dioon edule
• 
  Male cone of Dioon mejiae
• 
  cone of Dioon merolae

At the beginning of the Cenozoic, about 65.5 million years ago, the distribution area of Dioon probably included most of North America. Fossil found in northern Alaska from the Eocene (33.9 - 55.8 ka) indicate this. While the climate was still very warm at the beginning of this geological age, around 2.8 million years ago, the early ice age began with the icing of the North Pole, causing the distribution of Dioon to shrink very sharply.

In the Paleocene (55.8 - 65.5 ka) Dioon was next to Zamia and Anemia the only today neotropical genus that was still present in the flora of Alaska. During the Tertiary period, about 2.6 million years ago, Dioons died in almost all of North America with only a few relic collections remained in a small area in Central America, where today still the diversity center of the genus lies. A spread to South America was not possible because the two continents were not connected until about 3 million years ago. After that, the size of the seeds was the main obstacle to further spreading Dioon into the Caribbean and further south.

Classically, the family of Zamiaceae is divided into two subfamilies with four tribes. Dioon is the only genus in the tribe of the Diooeae, which together with the Encephalarteae forms the subfamily of Encephalartoideae. This classification was based on morphological considerations. A morphological study of relationships in fossil and extant taxa revealed the following cladogram:[17]

A molecular biology analysis of a fragment of 26S ribosomal DNA revealed a somewhat similar cladogram that would dramatically alter the structure of Zamiaceae.[17] Dioon would therefore be the basal genus of the family.

The genus is commonly divided into two groups by Norstog & Nichols of distinct morphology. The first includes D. mejiae, D. rzedowskii, and D. spinulosum, which are characterized by the greater size of their fronds, trunks, and cones. The second group contains D. califanoi, D. caputoi, D. edule, D. holmgrenii, D. merolae, D. purpusii, D. sonorense, and D. tomasellii, which are smaller overall, with shorter trunks, much shorter fronds, and smaller cones.[18]

A phylogenetic study from 1993 confirmed these clades with one difference - D. caputoi was assigned to the other clade

Species accepted:[3]

Image	Scientific name	Distribution
	Dioon angustifolium Miq.	Nuevo León, Tamaulipas
	Dioon argenteum T.J.Greg. & al.	Oaxaca
	Dioon califanoi De Luca & Sabato	Puebla, Oaxaca
	Dioon caputoi De Luca, Sabato & Vázq.Torres	Puebla, Oaxaca
	Dioon edule Lindl.	Nuevo León, Tamaulipas, Veracruz, Guanajuato, Chiapas, Hidalgo, Mexico State, Querétaro, San Luis Potosí
	Dioon holmgrenii De Luca, Sabato & Vázq.Torres	Oaxaca
	Dioon mejiae Standl. & L.O.Williams	Honduras
	Dioon merolae De Luca, Sabato & Vázq.Torres	Oaxaca, Chiapas
	Dioon purpusii Rose	Oaxaca
	Dioon rzedowskii De Luca, Sabato & Vázq.Torres	Oaxaca
	Dioon sonorense (De Luca, Sabato & Vázq.Torres) Chemnick et al.	Sonora, Sinaloa
	Dioon spinulosum Dyer ex Eichl	Veracruz, Oaxaca
	Dioon stevensonii Nic.-Mor. & Vovides	Michoacán, Guerrero
	Dioon tomasellii De Luca, Sabato & Vázq.Torres	Durango, Jalisco, Nayarit

The earliest fossil finds date back to the Eocene and were found on Kupreanof Island in Alaska. However, it is probable that Dioon dates back to the Jurassic (145.5 - 199.6 ka) or even further, since fossils of other, closely related but more developed cycads were already detected from this era.[19] In the Pleistocene (0.011784-1.8 ka) and Holocene (today -0.011784 ka) eras species have probably carried out several north–south migrations and have followed the warm climatic conditions.[20]

One theory is that the recent dioon species have evolved from a number of vicarious events in the early Cenozoic, with different species representing different ecological conditions. These events must have followed each other for a very short time, which precluded an accumulation of synapomorphies, meaning that not many features could be re-acquired.

The extant species of Dioon are attributed to three prototypes. D. mejiae is considered the most evolutionary advanced species of the genus. D. spinulosum and D. rzedowskii are attributed to a second prototype, which was probably better adapted to mesial conditions, such as areas of medium humidity, which enable them to establish on the Atlantic coast of Oaxaca. The remaining species presumably come from a third, to xerotic, dry areas, adapted to the original form.