The correct answer is C. pollen.
Coniferous plants are members of the group of plants that are known as the gymnosperms. This group also includes cycads and ginkgos, and they are characterized by the presence of a naked seed, which is not surrounded by tissue.
The gymnosperms are evolutionarily more advanced than all plants except for the angiosperms, the flowering plants. Within the gymnosperms, the Coniferophyta are the most abundant and diverse of all the members of this group.
Conifers are plants that are well-adapted to inhabiting cold and dry parts of the world. They are able to produce wood and bark from secondary xylem and phloem, which enables these plants to grow into very tall trees.
The coniferous plants produce the sex cells or gametes within special reproductive structures called cones. These are hard structures that provide protection for the developing gametes.
In pine trees, the male and female cones are very different although they are both produced on the same tree. Female cones are much larger and more complex in size than the male cones.
Female gametes, megaspores that develop into ovules are produced within the female cones. The male gametes, the microspores are formed in the microsporophylls of the male cone.
These gametes form pollen grains which are dispersed by the wind. Some pollen lands on the female cones where they become glued to a sticky substance the ovule has produced. As this evaporates the pollen is drawn into the ovule where fertilization occurs.
The embryo that is formed develops a seed coat that protects it from desiccation. The seed is also light and able to be dispersed by the wind and various animals.
Conifers are plants that are within the group of plants that are known as the gymnosperms. Besides the conifers that are in the Coniferophyta, the gymnosperms also include the Ginkgophyta, Cycadophyta, and Gnetophyta.
Gymnosperms are characterized by the presence of a naked seed. This is the first time that a seed is evident in the evolution of plants. This seed is considered naked compared with the flowering plants because it is not surrounded by tissues.
In the flowering plants, seeds are often contained within a fruit, and plants are well adapted for the dispersal of seeds by animals. This is not the case in gymnosperms such as conifers.
Coniferous plants are the most abundant of all the gymnosperms, and they have adapted to also be able to survive and grow in cold and dry conditions.
They can survive dry conditions because they do not require the presence of water to reproduce, and in fact, they are second only to the flowering plants, the angiosperms, in being evolutionarily advanced.
The vascular tissue of conifers is also more advanced than the more primitive plants. They are also able to produce secondary growth in the form of wood and bark which provides great protection and support to the plant body.
This secondary tissue is produced from vascular cambium which develops into secondary xylem and phloem which also allows for an increase in the width of the plant. This advancement has enabled these plants to be able to grow very tall and colonize inhospitable environments.
Conifers also frequently have small, thin leaves called needles that are designed to not break under the weight of snow. This is the perfect adaptation for a plant that lives in regions of the world that receive snowfall every winter.
A good example of a conifer is the pine tree, which is common in many parts of the world. The conifers are so successful that vast coniferous forests are found in the temperate regions of the earth.
The life cycle of the conifers
The conifers have their reproductive parts enclosed in protective structures that are called cones. These are hard and tough structures in which the gametes are manufactured.
The female and male sex cells (gametes) are formed in different and separate cones that occur on the same tree. There is often a noticeable disparity in the size and structure of these two cones.
Female gametes, or megaspores, are formed inside female cones that are considerably larger in size than male cones, in the case of pine trees. The female sex cells develop into the eggs or ovules, the female gametophyte, of the pine tree.
The male cones are much small in size and it is within this structure that the microscopes or male sex cells are produced within the microsporophylls. The microspores divide to form the male gametophytes which are the pollen grains that are dispersed by the wind.
During the breeding season, several million pollen grains are carried by air currents, and some of these do find their way to female cones of neighboring pine trees.
When a pollen grain reaches a female cone they become stuck to a substance known as a pollen droplet. This is a secretion of the ovules which is sticky and extends some distance from the actual ovule itself.
Over time this secretion starts to evaporate and slowly it pulls the pollen closer to the ovule. The pollen also produces a tube and sperm cell that will eventually fuse with the ovule during fertilization.
However, fertilization takes a long time and in fact can take several months to occur, often as long as 15 months in pine trees.
Once the ovule has been fertilized the zygote undergoes several mitotic cell divisions. In pine, several embryos are usually formed (polyembryony), but only one typically survives to develop further.
A seed coat also forms around the surviving embryo to form a protective structure. The embryo inside the seed forms a shoot apex, a root apex, and cotyledons.
The shoot apex will develop into the shoot and the root apex into the root when the plant germinates. The cotyledons provide a source of food for the embryo.
Pine seeds are light and are easily dispersed by the wind and by the activity of various animals. The sporophyte stage is the pine plant that germinates from the seed.
- Editors of Encyclopedia Britannica (2018). Pine. Retrieved from Encyclopedia Britannica.
- T Delevoryas (2018). Gymnosperms. Retrieved from Encyclopedia Britannica.
- Editors of Encyclopedia Britannica (2018). Conifer. Retrieved from Encyclopedia Britannica.
- PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants, 4th edition. New York: USA, Worth Publishers.
- N Herr (2007). Life cycle of conifers. Retrieved from csun.edu.