Definition, Habit, habitat, distributions, classification and Characteristics of Gymnosperms

Gymnosperms are a group of plants that produce seeds not enclosed within the ovary or fruit. Gymnosperms are seed plants that do not produce flowers. The term gymnosperm was given by Theophrastus in his book “Enquiry into Plants”(300 BC).It is derived from two Greek words, ―gymnos” means naked and “sperma” means seeds. Gymnosperms and angiosperms are two groups of seed plants(Spermatophyta). The former are naked seeded plants in which ovules are not enclosed by ovary wall, born freely on open megasporophylls, whereas in later, ovules are born in the closed megasporophylls and are completely enclosed within the ovary. Due to this reason(protected seed) angiosperms are considered to be advanced than gymnosperms. As gymnosperms do not possess ovary, hence they don‘t produce fruits, whereas angiosperms possess ovary and produce fruits.

Definition, Habit, habitat, distributions, classification and Characteristics of Gymnosperms

Habit and habitat of gymnosperms :-

The gymnosperm is represented by a perennial, evergreen, woody plant. Most of them are trees and some are shrubs. A few gymnosperms may be lianas or climbers. There are no herbs. Gymnosperms include one of the world's tallest trees - Sequoia sempervirens (the Giant Red Wood Tree) measuring about 125 meters in height and 30 meters in girth.

Living gymnosperms are distributed worldwide, with a majority, particularly the conifers, in temperate and subarctic regions. Cycads and gnetophytes are mainly tropical to subtropical.

Distribution of Gymnosperms:

Gymnosperms occur mostly in colder parts of northern hemisphere  where they form extensive forests. Cycads occur in tropical and subtropical  areas. Welwitschia grows in African desert. Ephedra grows in arid areas . A number of gymnosperms are now grown as ornamentals e.g. Ginkgo and species of Thuja, Araucaria,

There are 721 species of gymnosperms found in the world. They are;

Ginkgophyta = 1 species

Coniferophyta = 550 species

Cycadophyta = 100 species

Gnetophyta = 70 species

6 species of gymnosperm are found in bangladesh.  

Classification of Gymnosperms according to Christenhusz ( 2011):

Christenhusz et al., (2011) proposed a new classification and linear sequence of the extant gymnosperms based on molecular and morphological phylogenetic studies. They divided all extant gymnosperms into 4 sub classes, 8 orders and 12 families as follows: 

Sub Class I. Cycadidae  

Order A. Cycadales

Family 1. Cycadaceae

Family 2. Zamiaceae

Sub Class II Ginkgoidae

Order B. Ginkgoales

Family 3. Ginkgoaceae

Sub Class III Gnetidae

Order C. Welwitschiales

Family 4. Welwitschiaceae

Order D. Gnetales

Family 5. Gnetaceae

Order E. Ephedrales

Family 6. Ephedraceae

Sub class IV Pinidae

Order F. Pinales

Family 7. Pinaceae

Order G. Araucariales

Family8. Araucariaceae

Family9. Podocarpaceae

Order H. Cupressales

Family10. Sciadopityaceae

Family11. Cupreassaceae

Family12. Taxaceae

Characteristics of Gymnosperms: 

1. Saprophytic (2n), 

2. Heterosporic ( Megasporophyll, Microsporophyll), 

3. Heteromorphic type Alternation of Generation, 

4. Differentiated into root, stem and leaves, 

5. megaphyllous leaves,

6.  Lack vessel in xylem, 

7. Endosporic gametophyte,

8. mesarch bundle,

9. archegonia, 

10. slow growth rate, 

11. lack of vegetative reproduction, 

12. dispersal of seed limited, 

Detailed Characteristics of Gymnosperms:

1. Gymnosperms are naked seeded plants.

2. They all are woody, perennial, xerophytes and evergreen.

3. Morphologically there are two distinct lines, i.e.palm shape (Cycads) and conical shape (Conifers) plant body.

4. Plants possess tap root system but in some plants additional symbiotic relationship is found between roots and algae in coralloid roots (Cycas) and between roots and fungi in mycorrhiza roots(Pinus).

5. Plants may be monomorphic (having one kind of leaves) and dimorphic (two kinds of leaves).

6. Vascular cylinders of roots arediarch to polyarch. Xylem is exarch.

7. Secondary wood in stems may be manoxylic (Cycas) or pycnoxylic (Pinus).

8. Xylem lacks vessels and phloem is devoid of companion cells except Ephedrales. Xylem of stem is usually mesarch or endarch.

9. Leaves possess thick cuticle, sunken stomata, mesarch xylem, transfusion tissue and may possess resin passage or latex tubes.

10. Plants are heterosporous. Both dioecious and monoecious plants are found.

11. Male cones are usually short lived and smaller while female cones are long lived and larger than the male cones.

12. Microsporangia (male spore bearing organs) are borne on the lower surface of microsporophylls while megasporangia (ovules) are produced on the upper surface of the megasporophylls.

13. Pollination is anaemophilous, i.e. takes place by means of wind.

14. Zygote development is meroblastic, i.e. only the basal part develops into an embryo, the upper and middle parts do not participate in the formation of the embryo.

15. Mature embryo is divided into root, stem and leaves.

16. Polyembryony (development of several embryos in a seed) is common feature of Pinus.

17. In gymnosperms, endosperm develops before fertilization and is haploid.

18. Since ovules are naked and are not surrounded by ovary, true fruits like that of

angiosperms, are not formed.

19. The germination of seed is epigeal.

20. Alternation of generation is heterologous and sporophytic generation is dominant and

independent.

21. Economically gymnosperms are considered an important group which are the source of

food, timber, paper, pulp, resin and drugs.

22. The important drug ephedrine obtained from green branches of Ephedra is widely used in cold, asthma, bronchitis, etc.

Resemblances Gymnosperms with Pteridophytes

1. Both show heteromorphic alternation of generation.

2. In both sporophytes are differentiated into root, stem and leaves.

3. Both possess megaphyllous leaves.

4. Both possess a well marked vascular system.

5. Both lack vessels in the xylem and companion cells in the phloem.

6. Both have mesarch bundles in their leaves.

7. Many pteridophytes are heterosporous like the gymnosperms.

8. Both possess endosporic gametophyte.

9. In both, gametophytes are highly reduced.

10. The male gametes of Cycads and Ginkgo are motile like those of pteridophytes.

11. In both, female sex organs are archegonia.

12. In both, a distinct embryo develops after fertilisation.

13. Embryogeny is endosporic in many pteridophytes and all gymnosperms.

Differences between Gymnosperms and Pteridophytes

1. Gymnosperms are commonly large sized trees, shrubs or rarely climbers whereas pteridophytes are smaller in size and are herbaceous.

2. Gymnosperms have tap root system whereas pteridophytes have an adventitious root system.

3. The stems of gymnosperms are aerial whereas the stem of pteridophytes is mostly underground rhizome.

4. All gymnosperms are heterosporous whereas most of pteridophytes are homosporous.

5. In all gymnosperms pollen tube develops as a result of germination of pollen grain, while

in pteridophytes pollen tube is not formed.

6. In gymnosperms megaspores remain permanently in the megasporangium whereas in pteridophytes they are shed from the sporangium.

7. Megasporangium in gymnosperms is protected by an integument which is not found in pteridophytes.

8. Neck canal cells are absent in the archegonial neck of gymnosperms but are present in pteridophytes.

Resemblance of Gymnosperms with Angiosperms

1. Both have well developed and long lived sporoganous generation.

2. Both include trees and shrubs.

3. Sporophytes of both are differentiated into root, stem and leaves.

4. In both the groups root system is well developed.

5. The angiosperms and gymnosperms (except Gnetales) possess vessels in xylem and companion cells in the phloem.

6. Both have heterosporous sporophytes and endosporic gametophytes.

7. In both pollen grains grow into pollen tubes.

8. In most gymnosperms and all angiosperms the male gametes are non motile.

9. In both nucellus is surrounded by integument to form an ovule.

10. In both male gametophytes are highly reduced.

11. In both embryogeny is endosporic.

12. In both seeds develop from the ovule.

Differences between Gymnosperms and Angiosperms

1. Gymnosperms are not herbaceous whereas the majority of angiosperms are herbaceous.

2. Majority of gymnosperms are perennials whereas angiosperms may be annual, biennial or perennial.

3. In angiosperms, xylem possesses vessels and phloem has companion cells, whereas in gymnosperms (exceptGnetales)xylem is devoid of vessels and phloem lacks companion cells.

4. Gymnosperms are unisexual and may be monoecious or dioecious whereas angiosperms may be bisexual as well as unisexual, monoecious ordioecious.

5. Reproduction by vegetative means is very rare in gymnosperms whereas the method is very common in angiosperms.

6. The reproductive organs in gymnosperms are commonly called as cones or strobili whereas the reproductive organs in angiosperms are known as flowers.

7. In gymnosperms ovules are naked whereas in angiosperms they are enclosed within ovary.

8. Gymnosperms possess the archegonia whereas the angiosperms lack archegonia.

9. In gymnosperms endosperm develops before fertilization and is haploid (n) whereas in angiosperms it develops after fertilization and is triploid (3n).

10. Gymnosperms do not exhibit double fertilization which is commonly found in Angiosperms.

11. Polyembryony is mostly found in gymnosperms whereas it is not common in angiosperms.

12. In gymnosperms zygote shows free nuclear divisions whereas in angiosperms free nuclear divisions are absent.

13. In gymnosperms, seeds are exposed and fruits are not formed whereas in angiosperms seeds are enclosed in ovary and fruits are formed.