Reproduction of Gnetum
Reproduction of Gnetum has double fertilization as seen in angiosperms. Gnetum is a dioecious plant having its male and female reproductive structures produced in separate plants. Male plants produce male cones and female plants produce female cones.
Male and Female Cones in Reproduction of Gnetum
Both male and female cones are produced from axillary or terminal parts. These cones are arranged in pairs and are called inflorescence due to their resemblance to the inflorescence of angiosperms. Male and female cones are similar and are almost indistinguishable until they are matured. Both have a central axis divided into nodes and internodes.
Bracts are produced from the nodes and they are arranged in a whorl. These bracts fuse to form the cup-like cupule or collar. Each cone will have 10-20 cupules or collars. Due to short internodes, these cupules look closely arranged.
Male Cone or Strobilus
A Gnetum male cone will have up to 20 collars. Each cupule will have male flowers arranged in 3-6 rings. These strobilus may also have abortive ovules at the top. However, G.africanum has fertile ovules on the male strobilus, making it a bisexual inflorescence.
Young male cones have closely arranged collars on their axis. They gradually elongate as they mature. The apical part of the male cone will have sterile collars or naked flowers, depending on the species.
Male strobilus is short and ensheathed when young. They become elongated when mature and come out of the bracts to be visible.
Gnetum Male Flowers
Each male flower has a perianth, stalk, and anthers at the top. Each nather has numerous microspore mother cells inside the microsporangia. These microspore mother cells produce 4 haploid microspores each after its meiotic division.
Female Cones
Female cones of Gnetum are similar to male cones, except there is only a single ring of ovules at each collar or cupule. Of these, only a few ovules mature and enlarge. The upper collars do not produce ovules. Some multicellular uniseriate hairs protect these ovules.
Ovules
A mature ovule of Gnetum has a central nucellus made of soft cells. It has three layers of coverings.
- The innermost layer grows into an elongated micropylar tube or style. At the micropylar end, it is free from the nucellus and is not fused.
- The second layer is the outer integument and the outermost layer is the perianth. Both these layers have stomata, latex, and stone cells.
- The middle layer becomes stony and the perianth becomes the thick and fleshy seed.
Female Gametophyte
The female gametophyte develops inside the nucellus. Two to four large hypodermal cells having a big nucleus become the archesporial cells. They each divide into a parietal and sporogenous cell. The sporogenous cells become megaspore mother cells.
On average, 6-8 megaspore mother cells are formed in the nucellus. Each of these will develop a female gametophyte but only one will survive. All other cells will gradually degenerate. The surviving spore mother cell undergoes meiosis to form 4 haploid nuclei inside the cell. This is called coenomegaspore- a megaspore with 4 haploid nuclei.
Each nucleus potentially forms the female megaspore. These four nuclei undergo multiple divisions to form up to 1024 nuclei without wall formation. The female gametophyte with multiple nuclei enlarges. A central vacuole is formed and it pushes the cytoplasm and nuclei to the periphery.
A few nuclei near the micropylar region become female gametes or eggs. Cytokinesis occurs only in the lower region of the gametophyte.
Pollination
During pollination, the microspore falls on the micropyle and sticks to the liquid secreted by the ovule. This pollen is drawn towards the pollen chamber.
A pollen tube is developed inside the pollen chamber. The pollen tube contains two male cells and a tube nucleus. Later, the pollen tube pierces the female gametophyte at the micropylar end.
When the pollen tube enters the female gametophyte, one of the potential gamete cells becomes conspicuous by enlarging. This female gamete comes in contact with the pollen tube. Once the tube touches the female gamete, the wall dissolves to allow the male gametes to enter the egg cell.
Only one of the male gametes fuse with the egg cell. Sometimes, both the male gametes fuse with two egg cells to form two zygotes. Even after fertilization, the other egg cells may still be available as egg cells.
The diploid zygote thus formed will develop into an embryo inside the seed. This will get dispersed and fall on an appropriate substratum to germinate and grow into a new Gnetum plant.
References
- Abraham P C. Bryophytes, Pteridophytes, Gymnosperms & Paleobotany. 2000. St. Mary’s Books & Publications.
- https://ramsadaycollege.com/upload/eclassroom/Botany/GDM_Botany_Gnetum_sem2_03.pdf
