I. SYNONYMS: gnetophytes

II. NUMBERS: ~ 80 extant species.

III. PHYLUM CHARACTERISTICS

• A. Structure
  • Habit: The gnetophytes are seed plants with decussate (text with tooltip) In opposites pairs, each pair at right angles to each other. leaves. They tend to be dioecious (text with tooltip) Dioecious organisms have separate male and female individuals. with both staminate and ovulate strobili being compound.
  • Pollen (text with tooltip) The collective mass of grains produced within the anthers of flowering plants or the male cones of a gymnosperm. In all seed plants, pollen is generated by the development of a microspore into a microgametophyte. The germination of the pollen grain leads to the development of a pollen tube, which delivers two sperm or sperm nuclei to the egg in the ovule. In flowering plants, mature microgametophyte has only two cells, a tube cell and a generative cell. : Pollen walls are variable. The microgametophyte has 1 or 2 prothallial cells (text with tooltip) Prothallial cells are remnants of the vegetative microgametophyte in pollen grains. , a stalk cell (text with tooltip) One of the products of division of the initial cells in the pollen of some gymnosperms, said to be the homologue of the antheridial stalk. , 2 nonflagellate sperm and a tube cell (text with tooltip) A tube cell develops as part of the microgametophyte within the pollen grain. The tube cell directs the growth and development of the pollen tube, which carries the sperm to the egg in an ovule. .
  • Microstrobilus: Compound strobilus, bract (text with tooltip) A reduced leaf, especially the small, scale-like leaves associated with a flower or flower cluster. -like appendages surround the microsporangia (pollen sacs).
  • Seeds (text with tooltip) Unit of sexual reproduction in some plants. Formed when an ovule is fertilized and comprised of outer coat that encloses stored food and an embryo. : Ovules (text with tooltip) An ovule is a structure that contains the megagametophyte in seed plants. The megagametophyte remains within the megasporangium (the nucellus), which is surrounded by layers of integuments. After fertilization, the ovule develops into a seed. with extra integuments (text with tooltip) The outer covering of an ovule, which becomes the testa of the seed. (or reduced bracts) around the nucellus (text with tooltip) Central part of a plant ovule; contains the embryo sac. . The micropyle (text with tooltip) An opening in the integuments of an ovule that exposes part of the megasporangial wall (a chamber called a pollen chamber in gymnosperms). Thus, in gymnosperms, pollen enters the micropyle and germinates in the pollen chamber. However, because the micropyle is not exposed in flowering plants, their pollen germinates on the stigma. The pollen tube grows through the style, and enters the ovule through the micropyle. projects as a long tube. Archaegonia (text with tooltip) The female reproductive organ containing the egg. develop at the micropylar end of the megagametophyte. There is evidence for double fertilization. The embryo is dicotyledonous (text with tooltip) Having two cotyledons in the same seed. .
  • Megastrobilus: Compound.
  • Stems: Wood contains vessels (text with tooltip) A long tube of vessel elements connected by perforation plates. These are typical of the wood of flowering plants and gnetophytes. .
  • Leaves: Decussate (opposite). The leaves of Gnetum resemble those of dicots.
  • Roots: Like many of the conifers.
  • Life History of:
    • Ephedra

• B. Ecology: Ephedra and Welwitschia are adapted to arid conditions and occur in North America and South-western Africa, respectively. Gnetum is a vine or shrub which inhabits tropical forests. They have a fossil record which dates to the Permian.