Phylum: Gnetophyta.
This
group has three orders Ephedrales, Gnetales, and Welwitschiales, each with one
living genus, Ephedra, Gnetum, and Welwitschia,
respectively. Ephedra
has been chosen for detailed discussion because it is more readily available
for study in the temperate zones than are the other two genera.
Although they are allied with the gymnosperms, members of these orders
have very little in common with one another or with other gymnosperm groups.
Plants of this division have primitive vessels in the secondary xylem.
These vessels, in Ephedra,
arise by perforation of the circular bordered pits of the sloping terminal
walls of the tracheids. The fossil record of Gnetophyta is scanty.
Pollen attributed to a plant related to Ephedra has been found in the Upper Cretaceous strata (100 -65
million years before present) of Long Island, New York.
It had been found previously in Tertiary strata from other parts of the
world.
Order:
Ephedrales
The genus Ephedra has about 40 species. It is a xerophytic plant in warm-temperate latitudes. It appears as a shrubby plant, sometimes reaching 2 meters in height. The leaves are reduced to minute, bract-like organs that function only early in development, however they are megaphylls. The delicate branches are the chief photosynthetic organ of Ephedra.
Ephedra
sp.
Whole plant
Both
the microstrobili and the megastrobili of Ephedra
are compound structures, in which the sporogenous organs are appendicular, (
at the tip of the branch) and not cauline.
A sporophyll surrounds the integument of the ovule, suggesting the
development of a primitive carpel. The
literature contains repeated suggestions that the angiosperms evolved from
Gnetalean ancestors.
Examine
the preserved material of the life history of Ephedra.
Note: a) sporophyte shoot with a node and three leaves.
Ephedra
sp. Branch
b) the microstrobilus - with protruding microsporophylls, sterile bracteoles and microsporangia.
c) mature and immature megastrobili with protruding micropylar tubes and cone bracts.
Examine the slide of the microstrobilus L.S. Note the microsporangia on the protuberant microsporophylls; and microspores with ribbed exines.
Ephedra
sp.
Microsporangia
Ephedra
sp.
Megasporangia
Ephedra
sp.
Microsporangia