Gymnosperms:Definition
The term Gymnosperm was first used by Scientist Theophrastus in his book ‘Enquiry into plants’. The word Gymno means nacked and Sperma means seed. Due to the presence of nacked seeds, they are also called Nacked seed-bearing plants. the Gymnosperms are called Phaenerogams with ovaries. Gymnosperms are woody plants that can range in size from enormous trees to little shrubs. Sequoia sempervirens is as tall as a 36 story building. Perhaps the smallest Gymnosperm is a Cycad, Zamia pygmea whose fronds are only 4 or 5 cm long. Sequoiadendron giganteum, on the other hand, is the largest and oldest.
General Characteristics of Gymnosperms
1. Distribution: There are about 70 genera and 725 species of living Gymnosperms distributed throughout the temperate and tropical regions of the world.
2. Habitat: They are generally present in colder areas but a few are present in warmer i.e. tropical and sub-tropical areas e,g Cycas is found in tropical regions.
3. Habit: They can survive for several years so they are perennial and are woody. They may be shrubs, trees and some of them are woody climbers e,g Ephedra. There is no Gymnosperm which is annual and herbaceous.
4. Plant Body: The dominant phase is a diploid sporophytic phase. The sporophyte contains true roots, stems, and leaves. They are carnophytes. The gametophytic phase is reduced and the independent existence of male and female gametophytes is absent.
5. Roots: The root system is tap roots, it arises from radicles. The taproots may be branched, some branches are normal and some branches are associated with micro-organisms. It means two types of root branches are present. Thus roots show Dimorphic conditions.
Cycas roots are associated with BGA i.e. it is involved in nitrogen fixation e,g Anaebana, the roots are known as coralloid roots. The roots can grow in any direction i.e. they are insensitive to gravity such types of roots are known as Apogeotrophic roots. In pinus, roots are associated with fungi such types of roots are known as Mycorrhizal roots. They help in the absorption of nutrients especially phosphorous.
6. Leaves: The Dimorphic leaves are present i.e there are two types of leaves. Scaly and Foliage leaf. Scaly leaves are non-green their function is protection. Foliage leaves are green they are involved in photosynthesis. Foliage leaves can be simple or compound.
If Lamina is not divided the leaf is called simple leaf e,g Pinus leaf
If lamina is divided the leaf is called compound leaf e,g Cycas leaf.
The leaves can tolerate extreme conditions i.e. wind, temperature, and Humidity. The surface area is reduced. This will prevent water loss under xerophytic conditions.
In Gymnosperms lateral veins are absent, the special tissue is present and is called transfusion tissue.
6. Stem: The stem may be branched as well as unbranched. The unbranched is present in Cycas and the branched stem is present in Pinus and Cedrus. The pinus is branched and it arranges that larger branches or longer branches are at the base and shorter branches are towards the upperside thus giving the appearance of the cone. This is known as Excurrent Habit.
7. Vascular tissue: The vascular tissue is present i.e Xylem and Phloem are present. The xylem contains Tracheids, Fibres but vessels are absent. Only primitive vessels are present in Gnetales (Gnetum, Ephedra, Welweshtia).In Phloem, there is the presence of Sieve cells, Phloem fibers but sieve tubes are absent. Companion cells are absent. Certain helping cells are present i.e. Albuminous cells.
8. Sporophyll: In Gymnosperms, there is the presence of woody sporophyll. sporophyll is a specialized structure that bears sporangia. There are two types of sporophylls micro-sporophyll and megasporophyll. Microsporophyll will aggregate and form a male cone and pollen cone (compact strobilus). Megasporophyll will also aggregate and form a female cone, a seed cone (compact strobilus). The female cone is known as megasporangium and the male cone is a microsporangiate.
9. Wood: It represents the secondary xylem. There are two types of tissues. The primary tissue is formed in the plant from the beginning and the tissue that is formed later or mature stages is called secondary xylem.
If the wood is formed by the activity of one cambium ring such type of wood is called Monoxyllic wood e,g Pinus.
If the wood is formed by the activity of more than one cambium ring such type of ring is known as Polyxylic wood e,g Cycas.
If the wood is hard such type of wood is known as Pycnoxylic wood
If the wood is soft such type of wood is known as Manoxylic wood.
If the wood contains vessels it is called Heteroxylous wood
If the wood does not contain vessels it is called Homoxylous wood.
10. The Gymnosperms are mostly Di-cotyledons that vary from 2 in Cycas and 2-14 in pinus.
11. The Embryo development is meroblastic
12. The life cycle is Diplontic and sporic meiosis is present.
13. Plants may be dioecious e,g Cycas or monoecious e,g pinus and are heterosporous producing microspores and megaspores.
14. Pollination takes place by wind and pollen grains directly reach the ovules. The male gametes reach the female gamete by forming a tube i.e. siphonogamy to affect fertilization.
15. Polyembryony is common in Pinus.
16. Fruits are not formed due to the absence of an ovary.
17. Some common examples of Gymnosperms are Cycas, Zamia, Pinus, Ginkgo, Taxus, Gnetum, Ephedra, Welwitschia, Taxus, etc.