Gymnosperms: Definition, Characteristics, Classification

What is it about pine trees that makes them so fascinating? Of course, the pine tree’s woody cones add to its attractiveness and familiarity, but many of us are unaware that pine trees and other conifers are gymnosperms. People may also believe that gymnosperms are only made up of conifers. The enormous flowered angiosperms are undoubtedly more well-known among seed plants, but Gymnosperms are also worth investigating. The gymnosperm’s inception is thought to have occurred around the late Carboniferous period. Gymnosperms are thus the outcome of a genome duplication that occurred millions of years ago.

Four major groups within the gymnosperms are usually recognized, sometimes each considered its own phylum- Cycadophyta, Ginkgophyta, Gnetophyta and Pinophyta. Continue reading to know more on the topic of gymnosperms.

Gymnosperms Definition

“Gymnosperms are naked seeded plants, which evolved earlier than the flowering plants. They have their seeds exposed on the megasporophylls, i.e., carpels. Probably, they are the first surviving seed plants”. (Source: Arihant Biology Handbook)

The term gymnosperm has originated from a combination of two Greek words “Gymnos” means ‘naked’ and “Sperma” means ‘seed’, literally known as naked seed.

Gymnosperms Characteristics

Gymnosperms are seed-producing plants, but they do not produce fruits like angiosperms. They are a small group of plants with less than \(900\) species. The characteristic of gymnosperms is given below:

1. Occurrence: Gymnosperms are found all around the planet. Most of them are present in the temperate and subarctic regions of the northern hemisphere.

2. Size: Gymnosperms include only tall, perennial trees. The smallest gymnosperm is Zamia pygmaea, and the tallest is Sequoia sempervirens. Gnetum ula is a woody climber.

3. Plant body: The plant body is sporophyte and divided into root, stem, and leaves.

4. Habit: Gymnosperms are mainly evergreen and woody plants. Most of the gymnosperms are trees and shrubs. A few of them may be liana or climbers.

5. Life Span: As gymnosperms do not produce fruits or flowers, it takes a longer time in fertilization and development. For example, a pine tree can have a lifespan of over \(100\) years.

6. Roots: They have a fibrous root or taproot system.  

7. Stems: They are woody plants, so they have erect stems. Stems of gymnosperms can be either unbranched or highly branched.

8. Leaves: Leaves may be simple, needle-like (Pinus, Juniper, etc.) or compound like palms. (Cycas), with a thick cuticle.
There are two main types of leaves:
a. Foliage leaves are green and photosynthetic.
b. Scale leaves are on the nodes and short branches, brown in colour and non-photosynthetic.

9. Xylem: The xylem carries water and minerals to different parts of the plant and provides support. Vessels are absent in the xylem of gymnosperms. Gnetum has vessels in the secondary wood.

10. Phloem: In most of the cases, the sieve tube, companion cells are absent in the phloem. Instead, they have sieve cells to conduct food. Albuminous cells are present in pine in place of companion cells.

11. Flower: They do not produce flowers.

12. Microsporophyll: In gymnosperms, microsporophyll is hard leafy triangular clusters of male cones that bear microsporangia that contain microspores.

13. Megasporophyll: In gymnosperms, megasporophyll bears a female gamete, megasporangia containing only one megaspore.
a. Male and female cones may be borne on the same plant (Pinus, monoecious) or different plants (Cycas, dioecious).

14. Pollination: Pollen grains are carried from male cone to female cone by wind (Anemophily).

15. Gametophyte:
a. Male gametophytes are formed from haploid microspores. Male gametophytes or pollen grains are extremely reduced to a limited number of cells. 
b. Female gametophytes are highly reduced and are formed from haploid megaspore. There are integuments, so the ovules are called integumented megasporangium. All gymnosperms ( except Gnetum) are unitegmic.

16. Endosperm: They have haploid endosperms.

17. Seeds: The seeds are naked in gymnosperms and borne in cones that are not visible till maturity.

Classification of Gymnosprems

The Gymnosperms are further divided into \(4\) classes according to Kramer and Green (See Kubitzki, \(1990\)). The four classes of Gymnosperms are as follows:

1. Cycadopsida
2. Ginkgopsida 
3. Coniferopsida
4. Gnetopsida

1. Cycadopsida

a. These are represented by both fossil and living members. They originated in the Triassic period. Many of these are found in tropical areas.
b. They are woody, dioecious (separate male and female plants).
c. They have a single, typically unbranched trunk at the top of the plant. The leaves are borne together on top of the trunk.
d. The cones of Cycads are large, with many fertile leaves like sporophylls arranged into cones.
e. Their shape is beautiful, so they are often used as ornamental plants.
f. Examples: Cycas, Zamia.

Fig: Cycas

2. Ginkgopsida

a. Ginkgo biloba is the only living species of this class, so it is often called living fossils, and the rest of all species are extinct.
b. They possess fan-shaped leaves with dichotomous venation.
c. They are dioecious. Individual plants produce either male or female cones.
d. Example: Ginkgo.

Fig: Ginkgo

3. Coniferopsida

a. These are the most common and dominant, evergreen, and tall gymnosperms.
b. They have needle-shaped leaves.
c. Well adapted in drier climates and present at higher altitudes.
d. Sporophylls are arranged around a stem.
e. Cones are usually woody, sometimes soft, and fleshy.
f. They are dioecious.
g. Example: Pinus, Cedrus, Spruce, Firs.

Fig: Pinus

4. Gnetopsida

a. They include shrubs, small trees.
b. The external and internal characteristics of Gnetum is like angiosperms.
c. Their reproductive structures are almost like flowering plants.
d. Reproductive organs are borne in whorls or inflorescence.
e. They have vessels in the xylem.
f. Example: Gnetum, Ephedra.

Fig: Gnetum

Gymnosperms Reproduction

Gymnosperms are plants that do not bear flowers and fruits and true seeds.
(a) Gymnosperms reproduce mainly by both vegetative and sexual methods.
(b) They are heterosporous.
(c) Megasporangia are developed on megasporophylls.
(d) Microsporangia are developed on microsporophylls.
(e) Sporophylls aggregated to form cones or strobili.
(f) Microsporophylls bear male spores and are compactly arranged in a male cone or male strobili or microsporangiate cone. Microsporangia in these cones bear microspore mother cells. Male gametophytes are formed from haploid microspores. Male gametophytes or pollen grains are extremely reduced to a limited number of cells.
(g) Megasporophylls bear female spores and are compactly arranged in female cones or female strobili or megasporangiate cones. Megasporangia in these cones bear megaspore mother cells. Female gametophytes are highly reduced and are formed from haploid megaspore. There are integuments, so the ovules are called integumented megasporangium. All gymnosperms (except Gnetum) are unitegmic.
(h) In advanced gymnosperms like Gnetum, archegonia are absent, like in the angiosperms, which produce non-motile female gamete eggs or ovum.
(i) Pollination is carried by wind in all gymnosperms—pollen grains land on the female cone.
(j) Pollen tube develops from the pollen grain to initiate fertilization.
(k) The pollen grain divides into two sperm cells by mitosis. 
(l) One of the sperm cells unites with the egg cells present in the archegonium during fertilization.
(m) After fertilization, a diploid zygote is formed and becomes a seed.
(n) The diploid zygote divides by numerous mitotic divisions to form the diploid embryo, from which the future diploid sporophyte develops.
(o) Polyembryony (formation of many embryos within one ovule) is typical in Pines.

Fig: Reproduction in Gymnosperms

Gymnosperm Seeds

In gymnosperms, seeds are not covered by any outer covering like the ovary wall, as in angiosperms. Instead, the seeds are naked as there is no ovary, so they are called naked seeded plants. The seed undergoes a period of rest and then disperses over a vast area. The germination is epigeal.

Fig: Gymnosperm Seeds

Gymnosperms Life Cycle

(a) Gymnosperms show an alternation of generation.
(b) The diploid photosynthetic sporophyte generation is the dominant part of the life cycle.
(c) The gametophytic generation is dependent on the sporophyte and haploid.
(d) These show a diplontic life cycle.

Fig: Life Cycle of Gymnosperms

Examples of Gymnosperms Plants

A few examples of gymnosperms plants are:
1. Cycas
2. Pinus
3. Ginkgo
4. Gnetum
5. Ephedra
6. Juniperus
7. Spruce
8. Abies
9. Cedrus
10. Thuja

Gymnosperms Importance

Gymnosperms have both ecological and economical importance:

Ecological Importance

1. Gymnosperms are an excellent source of food and provide habitat to animals. Pinus gerardiana is used as food.
2. Gymnosperm plants prevent soil erosion.
3. Gymnosperms produce lots of oxygen that reduce the number of greenhouse gases.

Economic Importance

1. Gymnosperms are the primary source of paper pulp, lumber, turpentine, and resins. Pinus roxburghii is used to make paper pulp.
2. Gymnosperms are a source of fuel.
3. Gymnosperms are used for making medicines. Ephedra gerardiana is used for the treatment of asthma, bronchitis.
4. Gymnosperms provide a lot of timber for making furniture and other construction purposes. Cedrus deodar is used for making doors, boats, and railway sleepers.
5. Gymnosperm plants are also used for decoration and ornaments.
6. Taxol, an anti-cancer drug, is obtained from Taxus baccata.

Summary

This article clarifies that gymnosperms are ancient plants, and their existence is for millions of years. They are considered as the first plant to have seeds. However, they do not produce flowers or fruits; instead, the seeds of these plants do not have an outer covering or shell, so they are called naked seeds. These plants are mainly found in tropical climates. Conifers and cycads are considered the largest group of plants in gymnosperms. They provide many ecological and economic benefits to humankind.

Frequently Asked Questions

We have provided few frequently asked questions on gymnosperms here:

Q.1. What are \(3\) examples of Gymnosperms?
Ans: The three examples of Gymnosperms are Cycas, Pinus and Gnetum.

Q.2. What are the four main Gymnosperm groups?
Ans: The four main Gymnosperm groups are:
a. Cycadopsida
b. Ginkgopsida
c. Coniferopsida
d. Gnetopsida

Q.3. What are Gymnosperm plants?
Ans: Gymnosperms are naked seeded plants, which evolved earlier than the flowering plants. They have their seeds exposed on the megasporophylls, i.e., carpels. Probably, they are the first surviving seed plants.

Q.4. What is the oldest Gymnosperm in the world?
Ans: The oldest Gymnosperm in the world are the Bristlecone Pines.

Q.5. Do gymnosperms have fruit?
Ans: No, gymnosperms do not have ovaries. So, they cannot produce fruit.

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