Holmes 1986 divided Gymnosperms into Three classes Cycadopsida, Gnetopsida, and Coniferopsidabut also described some fossil ancestors in a separate class Progymnospermopsida in the book “ Outline of plant Classification”.
Cycadopsida or Pteridospermales
The Pteridospermales are popularly known as “Seed Ferns”. They possessed fern-like foliage that bore seeds and is therefore called Pteridosperms. They first appeared on the planet in upper Devonian times of the Palaeozoic era. They were at their climax in the carboniferous period and became extinct in the Jurassic period of the Mesozoic era.
General characters of Cycadopsida
1. The stems of the plants are erect, thin, or weak.
2. The leaves are frond-like, large, and pinnately complex.
3. The Primary xylem was mesarch represented by solid or medullated protostele.
4. The secondary wood was manoxylic.
5. Tracheids possessed multiseriate pits on their radial walls.
6. The cortex was well developed and had a longitudinally aligned fibre strand.
7. Megasporophylls resembled foliage leaves or specialized structures and were occasionally peltate.
8. A well developed vascular supply was present in the seed
9. A distinct pollen chamber was also included in the seeds.
10. The common genera and species include Gangamopteris buriadica, G. cyclopteroides, Vetebraria callipteridium, Pecopteris concinna, etc
Gnetopsida or Gnetaceae
Gnetales are “a minor group of gymnosperm-like plants,” according to Foster and Gifford (1959). While Maheshwari and Vasil stated that “The plant order formally included three genera, Ephedra, Welwitschia, and Gnetum, which were considered to be the most evolved among the gymnosperms. However, Ephedra, Welwitschia, and Gnetum are distinguished by the presence of bare ovules as well as the absence of real style and stigma.
General Characters of Gnetaceae
Gnetales are highly evolved Gymnosperms that have the following traits and are thought by some botanists to be the ancestors of blooming plants or angiosperms.
1. Some of these plants are trees (Gnetum gnemon), while others are lianes or shrubs, and a couple is stumpy turnip-like.
2. Simple elliptical or strap-shaped leaves, or even tiny scales, are present. They are generally opposite or whorled.
3. Vessels can be found in secondary wood.
4. Flowers are unisexual, dioecious in most cases, and monoecious in a few Gnetum species.
5. An antherophore with one to eight synangia is found in each male flower.
6. Flowers are arranged in compound strobili or inflorences.
7. At the time of fertilization the pollen tube contains two male nuclei.
8. Example Gnetum, Ephedra, Welwitschia.
Coniferopsida or Coniferales
There are around 54 living genera and over 500 species in the Coniferales, which are the world’s main forest producers. According to Foster and Gifford, the order Coniferales has the most dominant and conspicuous gymnosperms in present floras.
General Characters of Coniferales
1. The sporophytes are densely branching trees or bushes, and the plant body is sporophytic. They are mostly evergreen and xerophytic, except Larix, Metasequoia, and Taxodium, which are deciduous.
2. The growth habit varies from incredibly large trees like sequoia to microscopic forms that are only a few millimetres tall.
3. The branches might be monomorphic or dimorphic, as in the case of pinus.
4. Vessels are absent
5. The leaves are of two types i.e. foliage leaves and scaly leaves. They are usually spirally arranged and only in a whorled or opposite pattern. Occasionally the leaves are broad.
6. Plants are either Monoecious or Dioecious.
7. Reproductive organs are unisexual cones.
8. The sporophylls are usually grouped in the shape of cones, thus they are given the popular name conifer.
9. Pollen grains can be winged(Pinus) or unwinged (Taxodium). They are wind-dispersed.
10. The male gametes are motile.
11. Pollination is anemophyllous.
12. Seeds are endospermic and winged with hard teste.
13. Two to many cotyledons are present in the embryo.
14. Examples Pinus, Abies, Cedrus, Cupressus, Juniperus, etc.
Pro-Gymnosperms
Some palaeozoic era fossils from the Devonian and Carboniferous periods were previously assumed to belong to Pteridophytes, but are now thought to be closer to gymnosperms, but not fully gymnosperms. Beck in 1960 identified linkages between two such fossils, archaeopteris fern-like fronds and callixylon gymnosperm-like trunks, these members were classed as the progymnospermopsida, a class of gymnosperms. Progymnosperms are thought to be the origin of both Cycadales and Coniferales by some Palaeobotanists. After the discovery of Rhynie flora and Pteridosperms, Stewart views the discovery of progymnospermopsida to be a watershed event that has improved our understanding of vascular plant evolution more than any other.