32.2: Plant Reproductive Development and Structure - Sexual Reproduction in Gymnosperms
- Describe the process of sexual reproduction in gymnosperms
Sexual Reproduction in Gymnosperms
As with angiosperms, the life cycle of gymnosperms is also characterized by alternation of generations. In conifers such as pines, the green leafy part of the plant is the sporophyte; the cones contain the male and female gametophytes. The female cones are larger than the male cones and are positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. Because the pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.
Male Gametophyte
A male cone has a central axis on which bracts, a type of modified leaf, are attached. The bracts, known as microsporophylls, are the sites where microspores will develop. The microspores develop inside the microsporangium. Within the microsporangium, cells known as microsporocytes divide by meiosis to produce four haploid microspores. Further mitosis of the microspore produces two nuclei: the generative nucleus and the tube nucleus. Upon maturity, the male gametophyte (pollen) is released from the male cones and is carried by the wind to land on female cones.
Female Gametophyte
The female cone also has a central axis on which bracts known as megasporophylls are present. In the female cone, megaspore mother cells are present in the megasporangium. The megaspore mother cell divides by meiosis to produce four haploid megaspores. One of the megaspores divides to form the multicellular female gametophyte, while the others divide to form the rest of the structure. The female gametophyte is contained within a structure called the archegonium.
Reproductive Process
Upon landing on the female cone, the tube cell of the pollen forms the pollen tube, through which the generative cell migrates towards the female gametophyte through the micropyle. It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte. The male gametophyte containing the generative cell splits into two sperm nuclei, one of which fuses with the egg, while the other degenerates. After fertilization of the egg, the diploid zygote is formed, which divides by mitosis to form the embryo. The scales of the cones are closed during development of the seed. The seed is covered by a seed coat, which is derived from the female sporophyte. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place because gymnosperm seeds have no covering.
Angiosperms Versus Gymnosperms
Gymnosperm reproduction differs from that of angiosperms in several ways. In angiosperms, the female gametophyte in the ovule exists in an enclosed structure, the ovary; in gymnosperms, the female gametophyte is present on exposed bracts of the female cone and is not enclosed in an ovary. Double fertilization is a key event in the life cycle of angiosperms, but is completely absent in gymnosperms. The male and female gametophyte structures are present on separate male and female cones in gymnosperms, whereas in angiosperms, they are a part of the flower. Finally, wind plays an important role in pollination in gymnosperms because pollen is blown by the wind to land on the female cones. Although many angiosperms are also wind-pollinated, animal pollination is more common.
Key Points
- In gymnosperms, a leafy green sporophyte generates cones containing male and female gametophytes; female cones are bigger than male cones and are located higher up in the tree.
- A male cone contains microsporophylls where male gametophytes ( pollen ) are produced and are later carried by wind to female gametophytes.
- The megaspore mother cell in the female cone divides by meiosis to produce four haploid megaspores; one of the megaspores divides to form the female gametophyte.
- The male gametophyte lands on the female cone, forming a pollen tube through which the generative cell travels to meet the female gametophyte.
- One of the two sperm cells released by the generative cell fuses with the egg, forming a diploid zygote that divides to form the embryo.
- Unlike angiosperms, ovaries are absent in gymnosperms, double fertilization does not take place, male and female gametophytes are present on cones rather than flowers, and wind (not animals) drives pollination.
Key Terms
- megasporophyll : bears megasporangium, which produces megaspores that divide into the female gametophyte
- microsporophyll : a leaflike organ that bears microsporangium, which produces microspores that divide into the male gametophyte (pollen)