Exploring the Fascinating World of Ginkgoales: Living Fossils and Ancient Lineages

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https://en.wikipedia.org/wiki/Ginkgo_biloba

Introduction to Ginkgoales

The order Ginkgoales, represented today solely by Ginkgo biloba, once flourished with diverse species across the globe during the Triassic period approximately 200 million years ago. However, all genera, except Ginkgo biloba, are now extinct.

Varieties of Ginkgo biloba

According to Dallimore and Jackson (1948), G. biloba is represented by five varieties:

• Ginkgo biloba var aurea (Nelson) Beisson
• Ginkgo biloba var. fastigata Henry
• Ginkgo biloba var. paciniata Carriere
• Ginkgo biloba var. pendula Carnere
• Ginkgo biloba var. variegata Carriere

Geological History

The geological history of Ginkgoales reveals its emergence during the Permian period, as evidenced by records from the Late Paleozoic of India. The order achieved widespread distribution during the Triassic and Jurassic periods, but began declining during the Cretaceous.

Present Status

Today, Ginkgo biloba remains the sole living member of Ginkgoales, primarily found in parts of Southern and Eastern China. Records of this order are also noted from the Late Paleozoic of India, including species such as Ginkgophyton, Psygmophyllum, and Rhipidopsis.

Significance of Ginkgoales

Seward (1938) hailed Ginkgo as "one of the wonders of the world," emphasizing its remarkable persistence with little change over ages. Arnold (1947) highlighted Ginkgo biloba as one of the oldest living plants and possibly the oldest living genus of seed plants.

General Characteristics

1. Tree Structure: Tall, well-branched trees with short and long shoots, although early fossil members lacked them.
2. Wood Composition: Pycnoxylic.
3. Leaf Morphology: Large, leathery, fan-shaped, or strap-shaped, often deeply divided.
4. Venation: Dichotomous venation is common.
5. Male Organs: Unbranched, catkin-like, axillary position, bearing micro-sporangiophores.
6. Microsporangia: 2–12 pendulous microsporangia per microsporangiophore.
7. Spermatozoids: Motile, with spiral bands of flagella.
8. Ovules: Terminal on axillary axes, 2–10 in number.
9. Seeds: Large-sized, with fleshy outer and stony middle layers.

Classification

Formerly classified under Coniferae, Ginkgo was later placed in the order Ginkgoales by Engler. Florin (1936) included various genera like Baiera, Ginkgodium, and Ginkgoites in Ginkgoales, but their validity is debated. Sporne (1965) divided Ginkgoales into Trichopityaceae and Ginkgoaceae.

Phylogeny

Ginkgo biloba, the only living representative, exhibits unique characteristics:

• Bilobed, fan-shaped leaves
• Ovules with a collar at the base
• Microsporangiophore with a hump-like outgrowth
• Presence of a tent pole at the tip of the female gametophyte
• Absence of suspensors in the embryo.

Resemblances

Despite its unique traits, Ginkgoales shares similarities with Cordaitales, Pteridospermales, Filicales, Cycadales, and Coniferales, indicating evolutionary connections.

Resemblances with Cordaitales

(i) Presence of a double leaf trace, (ii) Endospermic beak in the mature ovule, and (iii) Probable motility of the spermatozoids.

Resemblances with Pteridospermales

(i) Presence of a leaf gap in the stem, (ii) Dichotomous open venation in leaves, (iii) A distinct pollen chamber, (iv) A massive fleshy layer in the ovules, and (v) Collar at the base of the ovules.

Resemblances with Filicales

(i) Similarities in primary xylem structure, secondary xylem structure, and periderm; (ii) Dichotomous open venation in leaves; (iii) Multi-flagellated and motile spermatozoids; and (iv) Presence of ventral canal cells in archegonia.

Resemblances with Cycadales

(i) Presence of multi-flagellated spermatozoids; (ii) Well-developed nucellar beak and pollen chamber; (iii) Haustorial nature of pollen tube, (iv) Presence of large egg, massive female gametophyte, and well-developed venter, (v) Endoscopic embryo with two cotyledons, (vi) Seed with thick and well-developed integument, and (vii) Hypogeal type of seed germination.

Despite these similarities, there are major differences between Ginkgoales and Cycadales, including: (i) Structure of reproductive organs, (ii) Vascular supply of ovules, (iii) Development of male gametophytes, (iv) Branched nature of stems in Ginkgoales, and generally unbranched in Cycadales, (v) Simple leaves in Ginkgoales, and compound leaves in Cycadales.

Resemblances with Coniferales:

Ginkgo shares several characteristics with many members of Coniferales, leading to the consideration of Ginkgoales as well-defined within Coniferophytes. Florin suggested that Ginkgoales, Coniferales, Cordaitales, and Taxales belong to the same natural group known as Coniferopsida. These four orders of Coniferopsida are believed to have evolved parallelly and separated from each other during the Upper Devonian or Carboniferous periods. Some major characteristics shared by both Ginkgoales and Coniferales include:

(i) General cone-like appearance of the tree, (ii) Extensively branched stem with long shoots and dwarf shoots, (iii) Simple leaves with sunken stomata, (iv) Narrow cortex and pith, with pycnoxylic wood, (v) Well-developed secondary wood, (vi) Uniseriate medullary rays, (vii) Circular bordered pits arranged uniseriately along the radial walls of tracheids, (viii) Longitudinal dehiscence of microsporangia, and (ix) Sessile nature of ovule.

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