Gymnosperms: Introduction

With an expert from BSI Dr. R. C. Srivastava kindly agreeing to coordinate Gymnosperms Fortnight should be a very useful one. 

     Gymnosperms constitute an interesting group of plants, usually shrubs or trees which produce seeds but don’t have fruits like flowering plants as ovules (developing seeds) are naked and not enclosed in an ovary. Here are some details from my book:

“Gymnosperms comprise a small group of seed plants characterized by naked seeds (gymno- naked, sperms- seeds) and absence of vessels (except Gnetopsids), endosperm formation independent of fertilization and commonly resulting in halploid endosperm (absence of double fertilization), absence of sieve tubes and companion cells. Group is represented by nearly 15 families, 80 genera and nearly 820 species, mostly consisting of evergreen trees and shrubs, distributed worlwide and forming extensive forests in North America, Europe and Asia. They represent some of the largest (Sequoiadendrod giganteum of California), tallest (Sequoia sempervirens of California and Oregon) and longest living (Pinus aristata) organisms in the world. 

Gymnosperms are woody trees or shrubs, herbaceous plants being absent from the group. The plants have well-developed tap root system, sometimes with symbiotic nitrogen fixing cyanobacterium (coralloid roots of Cycas) or mycorrhizae (Pinus). Vascular cylinder has xylem with tracheids with bordered pits and phloem with sieve cells. Leaves lack lateral veins (except in Ginkgo with dichotomous veins and Gnetum) , but are compensated by transfusion tissue. Sporangia are heterosporous, microsporangia and megasporangia borne on microsporophylls and megasporophylls, respectively; latter often arranged in distinct cones. Each microsporangium produces numerous microspores arranged in tetrads, since each microspore mother cell undergoes meiosis to produce four haploid microspores. Microspore  nucleus undergoes repeated divisions to form male gametophyte, which develops wall to become a pollen grain. The megasporangium, known as ovule, on the other hand, develops a single megaspore mother cell, surrounded by nucellus and integument, with an opening known as micropyle, at the end of integument. Of the four haploid megaspores resulting after meiosis, three degenerate, and only one megaspore is functional. Latter, after repeated nuclear divisions and wall formations produces a female gametophyte with several archegonia, consisting of an enlarged egg cell and two or four neck cells. Pollen grains of gymnosperms are carried by wind, land on micropyle and adhere to sticky fluid released by the female gametophyte. The pollen germinates to produce a pollen tube, that grows through nucellus and releases two sperms. One fuses with the egg to form zygote after fertilization. Latter develops into an embryo within matured ovule known as seed.”

The paper  by Dr. R. C. Srivastava: “Diversity and economic importance of living gymnosperms in India“ National Academy Science Letters 2006 Vol. 29 No. 3/4 pp. 75-84 gives following in the Preface:

Ginkgoaceae

Taxodiaceae

(These need to be shifted to Cupressaceae, or some from Cupressaceae shifted here if family is to be recognised). Recent treatments merge Taxodiaceae with Cupressaceae.

Podocarpaceae

Araucariaceae

Taxaceae

Ephedraceae

Gnetaceae

  

I am sure more taxa would be added after the completion of this episode.

---