After this process is completed, the individual sporophylls separate the cone breaks apart and float in the wind to a habitable place. This is concluded with germination and the formation of a seedling. Conifers have sperm that do not have flagella, but instead are conveyed to the egg via a pollen tube.
It is important to note that the seeds of gymnosperms are not enclosed in their final state upon the cone. Female cone of Tamarack pine : The female cone of Pinus tontorta , the Tamarack Pine, showing the rough scales. This is the cone that produces ovules. Male cone of Tamarack pine : The male cone of Pinus tontorta , the Tamarack pine, showing the close proximity of the scales.
This is the cone that produces pollen. Conifers are monoecious plants that produce both male and female cones, each making the necessary gametes used for fertilization. Pine trees are conifers cone bearing and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous, generating two different types of spores: male microspores and female megaspores.
In the male cones staminate cones , the microsporocytes give rise to pollen grains by meiosis. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly as the generative cell in the pollen grain divides into two haploid sperm cells by mitosis.
At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell. Female cones ovulate cones contain two ovules per scale. One megaspore mother cell megasporocyte undergoes meiosis in each ovule. Three of the four cells break down leaving only a single surviving cell which will develop into a female multicellular gametophyte.
It encloses archegonia an archegonium is a reproductive organ that contains a single large egg. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination.
The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte that will provide nutrients, and the embryo itself. In the life cycle of a conifer, the sporophyte 2n phase is the longest phase.
Learning Objectives Discuss the type of seeds produced by gymnosperms. Key Points Gymnosperms produce both male and female cones, each making the gametes needed for fertilization; this makes them heterosporous. Megaspores made in cones develop into the female gametophytes inside the ovules of gymnosperms, while pollen grains develop from cones that produce microspores. Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule.
Key Terms ovule : the structure in a plant that develops into a seed after fertilization; the megasporangium of a seed plant with its enclosing integuments sporophyll : the equivalent to a leaf in ferns and mosses that bears the sporangia heterosporous : producing both male and female gametophytes. Characteristics of Gymnosperms Gymnosperms are seed plants adapted to life on land; thus, they are autotrophic, photosynthetic organisms that tend to conserve water.
The sugar pine Pinus lambertiana produces the longest seed cone up to 25 inches of any conifers and is the tallest of the pines, reaching feet. Closer to Missouri, Scotch pine P. Other cone-bearing species, such as Eastern white pine P. Because Austrian pine P. Among the spruce Picea species, Norway spruce P. Even when young, Norway spruce trees produce cones.
These trees are fast-growing up to 3 feet per year for the first 25 years when planted in deep, well drained soils, with good moisture. Branches are pendulous and will produce mature female cones 5 to 7 months after pollination. The slow-growing, Colorado spruce P. Colorado spruce cones are produced in the upper part of the tree and are smaller than those of Norway spruce. Several Colorado spruce cultivars are available with blue-colored needles.
White spruce P. The scales of white spruce seed cones are thin, flexible, and have a rounded margin. The cones of fir trees Abies sp. Seed cones of fir trees can range from very small 1.
White fir A. White fir is recommended for planting in Missouri, and is often sold at cut-your-own Christmas tree farms. Cone-bearing species, such as Balsam fir A.
Douglas-fir Pseudotsuga menziesii is not a true fir, but produces one of the most interesting cones. Mature cones have broad rounded scales separated by another elongated bract with three projections of varying lengths Figure 2.
The megaspores will mature into eggs 1n. Learning Objectives Describe the life cycle of a gymnosperm. Key Points Male cones give rise to microspores, which produce pollen grains, while female cones give rise to megaspores, which produce ovules.
The pollen tube develops from the pollen grain to initiate fertilization; the pollen grain divides into two sperm cells by mitosis; one of the sperm cells unites with the egg cell during fertilization. Once the ovule is fertilized, a diploid sporophyte is produced, which gives rise to the embryo enclosed in a seed coat of tissue from the parent plant.
Fetilization and seed development can take years; the seed that is formed is made up of three tissues: the seed coat, the gametophyte, and the embryo. Key Terms megaspore : the larger spore of a heterosporous plant, typically producing a female gametophyte microspore : a small spore, as contrasted to the larger megaspore, which develops into male gametophytes monoecious : having the male stamen and female carpel reproductive organs on the same plant rather than on separate plants.
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