Q.3. Describe the resemblances and differences of gymnosperms.
Related Questions -
Q. Comment upon : Gymnosperms is connecting link between pteridophyta and Angiosperm. (2013)
Ans. Resemblance of Gymnosperms with Pteridophytes: -
The gymnosperms resemble pteridophytes in several features. These features are as follows -
1. Both show marked heterologous alternation of generation. In both sporophytic generation is dominant, independant and large at maturity while the gametophytic generation shows progessive reduction and dependance on sporophyte.
2. Both exhibit gradual increase in the complexity as regards external and internal features of the sporophytes:
(a) In both sporophytes are distinguished into root, stem and leaves.
(b) Both possess megaphyllous leaves. Many cycads resemble the ferns in having large compound leaves and in circinate vernation (ptyxis) of the leaves.
(c) Both possess a well marked vascular system.
(d) Both gymnosperms (except Gnetum, Ephedra and Welwitsctia) and pteridophytes lack vessels in the xylem and companion cells in the phloem.
(e) Both have mesarch bundles in their leaves.
(1) Both the gymnosperms and some pteridophytes (Isoetes and Botrychium) undergo secondary growth.
(g) Some gymnosperms (Ginkgo) and ferns show dichotomous venation.
3. Ancient gymnosperms (Cycadofilicales) were so much indentical with pteridophytes (ferns) that for a long time they were mistaken for ferns and were called “ferns bearing seeds”.
4. Many pteridophytes (Selaginella, Isoetes, Marsilea, Azolla and Salvinia) are heterosporous like the gymnosperms.
5. Some of the heterosporous pteridophytes, like the gymnosperms, possess one megaspore in the megasporangium.
6. Both gymnosperms and heterosporous pteridophytes possess endosporic gametophytes
7. In both gametophytes are highly reduced. The male gametophytes of heterosporous pteridophytes and majority of gymnosperms possess only two male prothallial cells.
8. Female prothallus (endosperm) develops before fertilization as a result of free nuclear divisions in the tissue of female gametophyte in both the groups.
8. The male gamets of cycads (Cycas) and Ginkgo are motile, like the pteridophytes.
10. In both the groups female sex organs are archegonia.
11. There develops a distinct embryo after fertilization in both the groups.
12. Embryogeny is endoscopic in many pteridophytes and all the gymnosperms.
13. Spore germination in both the groups is precocious.
Differences between Gymnosperms and Pteridophytes: -
Resemblances of Gymnosperms with Angiosperms: -
The gymnosperms show following resemblances with angiosperms-
1. Both have well developed and long-lived sporophytic generation.
2. Both include trees and shrubs and may be erect, scandent, or climbing.
3. The root system is well developed and the roots may be diarch triarch, tetrarch, or polyarch. The roots possess secondary growth. The xylem is exarch in the roots.
4. The stole of stem is eustelic. Secondary growth occurs in the stem. The secondary wood may be monoxylic or polyxylic.
5. Both plants group produce seed.
6. Both the groups are heterosporous and posses endosporic and reduced gametophytes .
7. The nucellus is surrounded by an integument to form a stucture called the ovule. The ovule has a micropyle and may be stalked or sessile.
8. The megaspore is in organic connection with the megasporangium and is never shed.
9. The pollen grain grow into pollen tubes.
10. The gametes are non-motile in majority of gymnosperms and all angiosperms.
11. The male gametophytes are highly reduced. Gnetum lacks prothallus cells.
12. The germination of spores is precocious.
13. The embryogeny is endosopic.
Differences between Gymnosperms and angiosperms: -
The above resemblances and differences between gymnosperms and other vascular plants suggest a close, affinity between them but also supports that gymnosperms are an independent assemblage of plants that have certain characters peculiar to them alone.