Division Gnetophyta

Division Gnetophyta contains three groups of enigmatic plants, each of which is often placed in its own order: Gnetum with 30 species (order Gnetales; Fig. 1), Ephedra with about 40 species (order Ephedrales; Fig. 2), and Welwitschia mirabilis, the only species in order Welwitschiales (Fig. 3; Table ).

FIGURE 1:Gnetum. (a) Plants of Gnetum strongly resemble dicots, having broad leaves and woody stems. (G. Davidse) (b) Ovules are not borne in cones. 

FIGURE 2:Ephedra. (a) Plants of Ephedra often occur in dry areas and strongly resemble many types of desert-adapted dicots. Although their reproductive structures are gymnospermous, the microsporangiate cone (b) could be mistaken for a staminate imperfect flower. The naked ovules reveal that they are not angiosperms (c). 

FIGURE 3:Welwitschia mirabilis. (a) Whole plant with torn leaves. (William E. Ferguson) (b) Microsporangiate strobili. (c) Megasporangiate strobilus. (Richard Shiell/Earth Scenes)

Gnetums are mostly vines or small shrubs with broad leaves similar to those of dicots. They are native to southeast Asia, tropical Africa, and the Amazon Basin. Plants of Ephedra are tough shrubs and bushes that inhabit desert regions in northern Mexico and south­western United States. Their leaves are reduced and scalelike. The few living plants of Welwitschia exist only in deserts of South Africa or in cultivation. They have a short wick stem and only two leaves, but the leaves grow perennially from a basal meristem, becoming increasingly longer.

All three genera are unusual in being gymnosperms with vessels in their wood. This had been thought to show that they might be related to primitive angiosperms. However, their vessel elements evolved from tracheitis with circular bordered pits, whereas those of angiosperms derived from scalariform tracheids. Furthermore, angiosperms are thought to have evolved from vessel-less ancestors, the vessels evolving after flowers, not before them.

Unlike the pollen cones of all other gymnosperms, those of gnetophytes are compound and contain small bracts. Seed cones are also compound and contain extra layers of tissue around the ovules; the tissue is variously interpreted as an extra integument, bract, or sporophyll.

The few fossils of gnetophyte organs or tissues are only several million years old, too recent to be of much help in understanding the evolution and ancestry of the group. The pollen is quite distinctive, being spindle-shaped and having narrow ridges. It is easy to recognize, and fossil pollen of this type occurs as far back as the late Triassic Period, but pollen has not helped reveal their origins either.

مواضيع ذات صلة

البذور الاصطناعية Artificial Seeds

مقارنة بين نباتات مغطاة البذور (النباتات الزهرية) ومعراة البذور

مغطاة البذور (النباتات الزهرية) Division Mangnoliophyta

الصنوبـر (Pinus)

النباتات معراة البذور Gymnosperms

النباتات البذرية

بذرة الفول

بذرة الخروع Caster OiI Seed

حبة الذرة Zea mays

دراسة مثال لإنبات بذور ذوات الفلقة الواحدة

دراسة بعض الأمثلة لإنبات بذور ذوات الفلقتين

التغيرات التي تحدث للبذور أثناء الإنبات