The following points highlight the nine characteristic features of living gymnosperms in a plant body. The features are: 1. Morphological Parts 2. Reproductive Organs 3. Ovule and Female Gametophyte 4. Microspores 5. Male Gametophyte 6. Pollination 7. Fertilization 8. Embryogeny 9. Formation of Seed.
Living Gymnosperms: Characteristics Features # 1.
Morphological Parts:
1. The living gymnosperms include middle-sized or tall trees and shrubs. There are no herbs and climbers.
2. Approximately, there are 70 genera and 725 species.
3. They do not reproduce vegetatively by means of cuttings, layering or budding, but vegetatively propagating structures like bulbils are known in some Cycads (e.g., Cycas).
4. The roots are generally tap roots but mycorrhizic (e.g., Pinus) and coralloid roots (e.g., Cycas) are also known.
5. The stems are aerial, erect, underground (Welwitschia), un-branched (Cycas), or branched (Pinus, Ephedra, etc.)
6. In Pinus, the branches may be of two types:
(i) The long shoots; and
(ii) the dwarf shoots that bear at their apices clusters of green leaves and are collectively known as spur.
7. Both the microphyllous and the megaphyllous leaves are known among the gymnosperms. The former are small and scaly, whereas the latter are large and well-developed and their vascular supply always leaves a leaf gap in the stem stele.
8. The pinnate leaves in Cycas persist for 3-10 years, and when they fall, the leaf bases persist even longer. Pinnate leaves of Cycas alone show circinate vernation. The pinnae in Cycas have a single mid-rib and there are no lateral veins.
9. The leaves are mostly evergreen and may possess resin passages (e.g., in Pinus).
10. The leaves of Gnetales lack resin passages.
11. The leaves of conifers and cycads possess a transfusion tissue. The stomata are sunken (e.g., Cycas, Pinus).
12. In Cycas, the stomata are confined on the lower epdermis alone. The stomata have guard cells usually surrounded by subsidiary cells whose number varies from 2-4 or even more.
13. The mesophyll may (Cycas) or may not (Pinus) be distinguished into palisade tissue and spongy parenchyma.
14. The leaves may be triangular (Pinus roxburghii), semi-circular {P. sylvestris) and bifacial (leaflets of cycas). The leaflets in Cycas revoluta have revolute margins. The leaves of Pinus kempferi are flattened.
15. The vasculature of the petiole of Cycas shows omega shaped (Ω) disposition of the vascular bundles. The bundles are open. A characteristic feature of the vascular bundles of petiole and lamina in Cycas is the diploxylic xylem. In this case there is a patch of centripetally developed metaxylem and two patches of centrifugally developed xylem.
16. The leaf traces in Cycas are endarch at their attachment in the pith region of the stem. They become meserch during their course through the cortex. In Ephedra the unilacunar nodes receive two leaf traces that have quite distinct points of origin. They lack cambiam.
17. The secondary wood in gymnosperms may be:
(i) Manoxylic or
(ii) Pycnoxylic.
The manoxylic wood is found in Cycadophyta and is porous, soft and more parenchymatous in nature. It has wide medullary rays and is useless commercially.
18. The pycnoxyllic wood is characteristic of Coniferophyta and is compact and has narrow medullary rays. It is of great commercial use. The xylem lacks wood vessels (except in Gnetales), whereas the phloem is devoid of companion cells.
19. The xylem is usually mesarch or endarch (exarch in roots).
20. The secondary vasculature may be monoxylic (where there is a persistent cambiam as in Pinus) or polyxylic where different layers of cambium appear and result in distinct concentric rings of secondary xylem and phloem (e.g., Cycas, Gnetum).
21. The meduallary rays may be uniseriate or multiseriate and may be homogenous (e.g., Ginkgo) or heterogenous (e.g., Pinus). The resin canals may also be present in the secondary wood of many gymnosperms (e.g., Pinaceae).
Living Gymnosperms: Characteristics Features # 2.
Reproductive Organs:
1. The gymnosperms are heterosporous, and the two kinds of spores produce two kinds of gametophytes, i.e., the microspores or the pollen grains produce male gametophytes, whereas the single megaspore enclosed within the megasporangium (nucellus) develops into a female gametophyte that bears two or more archegonia or female sex organs.
2. The two kinds of spores (microspore and megaspore) are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.
3. The strobili bearing microsporophyll’s and microsporangia are called microsporangiate or male strobili, whereas those bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
4. The two types of cones or strobili may be borne on the same tree (e.g., Pinus) or on different trees (e.g., Cycas, Ginkgo). The microsporangiam contains numerous microspores, whereas the megasporangiam contains only one megaspore.
5. The microspores and the megaspores are haploid and developed as a result of meiosis in the respective spore mother cells. They are the pioneer structures of the male and female gametophytes, respectively.
6. The gametophytes are endosporic, i.e., they develop within the respective spore wall.
Living Gymnosperms: Characteristics Features # 3.
Ovule and Female Gametophyte:
1. The ovules are naked and are borne on megasporophylls that are usually spirally arranged around a central axis. The ovules are sessile and those of Cycas are the largest in the plant kingdom.
2. The ovule consists of a parenchymatous mass of cells called the nucellus or the megasporangium. The nucellus encloses a single diploid megaspore mother cell that undergoes meiosis to form a linear tetrad of 4 haploid megaspores. Out of which only lower one remains functional, whereas the rest degenerate. The functional megaspore enlarges and undergoes free nuclear division to form large number of free nuclei.
This is the young female gametophyte which develops within the megaspore. By now the nucellus becomes enveloped by a single massive or integument that grows around bearing a small pore, the micropyle at one end. This is the micropylar end of the ovule.
Thus, integument megasporangium is called ovule. Now centripetal wall formation starts around the free nuclei of the female gametophyte. It continues till the whole female gametophyte becomes cellular. The ovule remains surrounded by a three layered integument.
Living Gymnosperms: Characteristics Features # 4.
Microspores:
1. In gymnosperms the sporophyte is the only visible individual, bearing the gametophytes, both male and the female. The male gametophyte or the pollen grain is mobile while the female gametophyte or the megaspore is quiescent and enclosed in a sporophytic vegetative envelope.
2. The microspores or the pollen grains are unicellular and haploid structure produced within microsporangia. In Cycas the microspores are tetrahedral in shape and show a distinct polarity because exine is thicker towards the base. They are uniaperturate. The microspores of Pinus are winged or saccate. The development of wings is considered to be a significant development during the course of pollen evolution.
Living Gymnosperms: Characteristics Features # 5.
Male Gametophyte:
1. The male gametophytes are endosporic and complete their development partly in the microsporangiam and partly in the pollen chamber of the ovule. In microgametophytes of Cycads there is one male prothallus cell that protrudes into a large sterile cell or the stalk cell followed by a body cell or a spermatogenous cell and a tube cell.
They are arranged in a linear row. The body cell divides into two multiciliate male gametes. The pollen tube is formed in all Cycads and is more haustorial in nature than a sperm carrier.
In Pinus the microspore nucleus divides twice by periclinal walls to cut off two male prothallus cells and an antheridial cell. The latter divides periclinally to form a generative cell and a tube cell.
Living Gymnosperms: Characteristics Features # 6.
Pollination:
1. The pollination in gymnosperms takes place by means of wind and results in the transference of semigerminated pollen grains on the micropyle of the ovule. These pollen grains, in most of the gymnosperms, are caught into a pollination drop secreted by the micropylar end of the ovule.
The semigerminated microspores are drawn into the ovule by the drying of the pollination drop. The micropyle closes after the act of drawing in of the microgametophyte.
Living Gymnosperms: Characteristics Features # 7.
Fertilization:
1. This process embraces all the changes that take place from the time of liberation of the male gametes from the pollen tube to their fusion with the female gametes. In Cycas the grain end of the pollen tube becomes extremely turgid and ultimately bursts to release its contents into the archegonial chamber.
The fluid of the pollen tube fills the archegonial chamber. In this fluid multi-ciliate sperms start swimming and find their way towards the archegonial necks. The phenomenon of fertilization by ciliated and mobile sperms in Cycas is called zoodigamy.
2. In the conifers (e.g., Pinus) the pollen tube acts as the carrier of male gametes. The male gamete pierces through the neck of archegonium and bursts to liberate its contents in the egg cell. This type of fertilization is called siphonogamous or siphonogamy. One of the male gametes fuses with the egg to form the zygote. This process is called syngamy.
3. In Ephedra the pollen tube grows through the archegonial neck and discharges its contents into the cytoplasm of the egg by bursting of its swollen tip. The larger male nucleus moves towards the female nucleus which is surrounded by a dense cytoplasmic sheath.
The male nucleus moves towards the female nucleus which is surrounded by a dense cytoplasmic sheath. The male nucleus moves through the sheath and fuses with the female nucleus to form a fusion nucleus which along with its surrounding cytoplasm is called zygote.
4. The extinct orders of gymnosperms, such as Cycadofilicales, Bennettitales, and Cordaitales did not produce pollen tubes and the motile sperms were liberated directly into the pollen chambers. This process is called zooidogamous.
Living Gymnosperms: Characteristics Features # 8.
Embryogeny:
1. In living gymnosperms the basic gymnosperm plan of embryo development can be divided into three stages:
(i). Free nuclear proembryo;
(ii). Primary proembryo or the celluler stage and
(iii). Direct development of primary proembryo into late embryo.
(i). Free nuclear proembryo:
The whole fertilised egg does not develop into an embryo. The divisions of zygote do not directly lead to the formation of an embryo (as found in angiosperms) but produce a structure called the proembryo. This can be defined as an ‘early embryonic stage beginning after the first division of the zygote and ending when a basal embryonic region begins to differentiate the distinctive organs of the embryo.
(ii). Primary proembryo or the cellular stage:
Beginning of this stage is marked by the appearance of evanescent segmentation of the cytoplasm and later formation of walls around the nuclei. This cellular primary proembryo shows no differentiation except into upper or micropyler and lower regions. The former is designated as the primary upper region while the latter is called the primary embryonal region.
(iii). Direct development of primary proembryo into late embryo:
The primary proembryo directly develops into the late embryo. In this region embryonal mass and pre-suspensor system are developed. Pre-suspensor system is regarded as the precursor of the suspensor system of conifers.
Living Gymnosperms: Characteristics Features # 9.
Formation of Seed:
1. Seed is the result of fertilisation and consequent enlargement of the ovule. The zygote develops into an embryo, the endosperm persists as a nutritive tissue, whereas the nucellus becomes disorganised or it may persist as dry tissue at the micropylar end of the seed. This is called the nucellar cap.
The inner fleshy layer may persist as a thin layer of seed coat called the tegmen. The middle stony layer changes into a hard layer called the testa, which protects the female gametophyte and the embryo.
The seed is a remarkable combination of two sporophytic generations and one gametophytic generation:
(i) The seed coat represents the old sporophytic generation.
(ii) The young embryo represents the new sporophytic generation.
(iii) The endosperm represents the gametophytic generation.
The germination of seed is epigeal, i.e., the cotyledons come above ground, in most of the genera. Vivipary has been reported in Ephedra trifurcata.
The alternation of generations is heterologous in all the gymnosperms.