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Affinities of Gymnosperms (Conifers)

Affinities of Gymnosperms

Gymnosperms constitute a heterogenous of plants groups with several evolutionary features which have been taking place since past to the present days. Evolutionary tendencies reveal that gymnosperms form a bridge between pteridophytes and angiosperms. Eames (1936) has rightly placed them between filicineae and angiospermae under the pteropsida, a subdivision of ptaschaeophyta. Keeping in view their resemblances with pteridophytes and angiosperms on one hand and certain differences with the same groups on the other hand they have been given place as an independent group of plants.

Similarities with Pteridophytes

Gymnosperms resemble pteridophytes in features mentioned below:

  1. Both gymnosperms and pteridophytes show marked heterologous alternation of generation. Sporophytic generation (2N) is dominant, independent and large at maturity while the gametophytic generation (N) shows progressive reduction and dependence.
  2. Sporophytes are distinguished into root, stem and leaves and possesses a well marked vascular system in pteridophytes and gymnosperms both.
  3. Absence of vessels in the xylem and companion cells in the phloem (except Gnetum, Ephedra and Welwitschia).
  4. In having mesarch vascular bundles in leaves.
  5. Pteridophytes like Isoetes and Botrichium resemble gymnosperms in undergoing secondary growth which although is not much pronounced.
  6. Presence of dichotomous venation in ferns and Ginkgo.
  7. Presence of megaphyllous leaves.
  8. Many cycads closely resemble ferns in having large compound leaves and circinate vernation. Ancient gymnosperms, Cycadofilicales (pteridosperms) were so much identical with pteridophytes (ferns) that for a long time, they were mistaken for ferns and were called “ferns bearing seeds”.
  9. Many pteridophytes (Selaginella, Isoetes, Marsilea, Azolla and Salvinia) are heterosporous and possess one megaspore in the megasporangium like the gynosperms, Germination of spore is precocious.
  10. Both heterosporous pteridophytes and gymnosperms possess endosporic gametophytes.
  11. Male gametes of cycads (Cycas) and Ginkgo are motile as those of the pteridophytes.
  12. Female sex organs are archegonia in both pteridophytes and gymnosperms.
  13. Male gametophytes in heterosporous pteridophytes and majority of gymnosperms are highly reduced and possess only two male prothallial cells.
  14. Endosperm (female prothallus) develops fertilizations as a result of free nuclear divisions in the tissue of female gametophyte in both ghmnosperms and pteridophytes.
  15. Embryogeny is endoscopic in many pteridophytes and majority of gymonsperms.

Differences Between Gymnosperms and Pteridophytes

  1. Gymnosperms differ from pteridophytes in the features mentioned below:
  2. Gymnosperms are commonly large sized trees, shrubs or climbers Pteridophytes, on contrary, are relatively smaller in size and are more commonly hygroscopic.
  3. Gymnosperms are characterized by the presence of tap roots whereas the roots of pteridophytes are adventitious.
  4. Gymnosperms possess aerial stem, whereas stems of pteridophytes (ferns) are mostly undergrounds (rhizome).
  5. Gymnosperms have eustelic organization, whereas in pteridophytes eustele is not found.
  6. Excessive secondary growth occurring in higher gymnosperms (conifers is a distinct anatomical advancement over the pteridophytes.
  7. In gymnosperms, male and female gametophytes are simple, non-green and dependent on the sporophytes. They are very much reduced in comparison to the gametophytes of pteridophytes.
  8. In all living gymnosperms pollen tube develops as a result of germination of pollen grains. Such a structure does not occur in the heterosporous pteridophytes.
  9. In gymnosperms, microspores, that are very light and wind dispersed, are shed from the sporophyte soon after their particle development and come to lie on the exuding nucleus from the micropyle. In pteridophytes, on the other hand microspores may or may not germinate in situ and male cells move in presence of water.
  10. In gymnosperms, megaspores remain in the megasporangium whereas in pteridophytes they are shed from the sporangia and develop into independent female prothallus.
  11. Archegonial neck in gymnosperms is very small and lacks neck canal cells and ventral canal cells Gnetum lacks archegonia. Archegonia are provided with neck canal cells and ventral canal cells in pteridophytes.
  12. Seen formation is not common in pteridophytes (except Selaginella) whereas in gymnosperms seeds are produced.

Similarities with Angiosperms

Gymnosperms resemble angiosperms in the features mentioned below:

  1. Sporophytic generation of both gymnosperms and angiosperms is well developed and long lived.
  2. Include trees and shrubs and may be erect, scandent of climbing.
  3. Plants are differentiated into root, stem and leaves.
  4. Roots exarch, diarch to polyarch and undergo secondary growth.
  5. Stem eustelic and undergo secondary growth resulting monoxylic or polyxylic wood.
  6. Presence vessels and companion cells (in gymnosperms only in Gnetales).
  7. Both gymnosperms and angiosperms are heterosporous plants. Germination of spores is precocious.
  8. Both the groups groups have endosporic and reduced nature of gametophytes.
  9. Pollen grains in both gymnosperms and angiosperms develop pollen tube.
  10. Male gametes are non-motile in majority of gymnosperms and all angiosperms.
  11. In both gymnosperms and angiosperms megaspore is in organic connection with the megasporangium.
  12. Megasporangium is never shed and is retained within the megaspoangium so as to form the seeds both gymnosperms and angiosperms.
  13. The nucleus in both gymnosperms and angiosperms is protected by an integument to form a structure called ovule. It may have a micropyle and may be sessile or stalked.

Differences between Gymnosperms and Angiosperms

Gymnosperms differ from the angiosperms in the features mentioned below:

  1. Majority of angiosperms are herbaceous in contrast to the gymnosperms which are mostly woody.
  2. Angiosperms may be annual, biennial or perennial whereas gymnosperms are usually perennial.
  3. In angiosperms, xylem has vessels and phloem possesses companion cells, whereas in gymnosperms (except Gnetales) xylem is devoid of vessels and phloem lacks companion cells.
  4. Gymnosperms rarely reproduce by vegetative means (Cycas by balbils) whereas this method is very common among the angiosperms.
  5. Reproductive organs in gymnosperms are commonly referred as cones or strobili (except the female reproductive organs in Cycas) that bear compactly arranged sporophylls around a central axis. Angiosperm flower is homologous to cone or strobilus but the reproductive organs do not occur compactly.
  6. Gymnosperms lack accessory attracting organs like nectarines, sepals and petals. These structures are present in angiosperm flowers.
  7. In gymnosperms cones are unisexual, whereas in angiosperms the flowers are usually bisexual.
  8. Pollen grains in gymnosperms give rise to prothallial cells during germination. In angiosperm, such prothallial cells are not found.
  9. In gymnosperm, the ovules are naked whereas in angiosperms they are enclosed by the carpels.
  10. Angiosperms lack archegonia (the female sex-organ) whereas the gymnosperms (except Gnetum) possess them.
  11. Gymnosperms exhibit anaemophilous pollination whereas among angiosperms anemophily entomophily, heterophilly and zoophilly are all prevalent.
  12. In gymnosperms, pollen grains directly fall on the micropyle during pollination, whereas in angiosperms they fall on the stigma, travel through style and finally reach ovary and ovule.
  13. In gymnosperms endosperm develops before fertilization and is a haploid (N) tissue. In contrast, in angiosperms, it is a triploid tissue (3N) that develops after fertilization.
  14. Gymnosperms, do not show double fertilization or triple fusion whereas in angiosperms. It is a characteristic phenomenon.
  15. Among gymnosperms cleavage polyembryony is prevalent, whereas in angiosperms it is absent.
  16. Zygote exhibits free nuclear divisions in gymnosperms but not in angiosperms.
  17. Angiospermous seeds are enclosed within the carpels or ovaries that develop fruits, whereas in gymnosperms the seeds are naked.

Above similarities and differences between gymnosperms and other vascular plants suggest a close affinity between them. The plants, however, have certain characters of their own which lead to firm and independent group gymnosperms occupying a position between pteridophytes and angiosperms.

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