Gymnosperms: General account, classification of Gymnosperms
Gymnosperms:-
> Gymnosperm:- “Gymnos” = necked and “Sperma” = seeds.
> The word Gymnosperm was first used by Theophrastus, a pupil of Aristotle in his famous book
“Enquiry into Plants”. He used this term in all those plants having unprotected (without covering) seeds.
General account:-
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:- These are green and photosynthetic.
b. Scale leaves:- These 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. 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:- They are formed from haploid microspores. Male gametophytes or pollen grains are extremely reduced to a limited number of cells.
b. Female gametophytes:- They 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 Gymnosperms:- Sporne (1965) classified gymnosperms into 3 classes, 9 orders and 31 families.
The classes include:
i. Cycadospsida
ii. Coniferopsida
iii. Gnetopsida
General Characters of Main classes:-
Class I – Cycadopsida:-
> Plants are palm-like or fern-like.
> Compound, frond-like pinnate leaves.
> Manoxylic wood.
> Sperms are motile.
> Flower like structures are absent. Strobili are simple.
> Example:- Cycas, Zamia.
Class II – Coniferopsida:-
> Tall trees with simple leaves of varied shape.
> Wood is pycnoxylic.
> Cone like strobili are present.
> Motile sperms are absent (except Ginkgo biloba).
> Example:- Pinus.
Class III – Gnetopsida:-
> Shrubs, trees and lianas.
> Leaves are elliptical or strap-shaped, simple, opposite or whorled.
> Motile sperms are absent.
> Wood contains vessels.
> Strobili is called as inflorescence.
> Flower like structure with perianth is present.
> Example:- Gnetum, Ephedra.