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Early land plants had sporophytes that produced identical spores (isosporous or homosporous) but the ancestors of the gymnosperms evolved complex heterosporous life cycles in which the spores producing male and female gametophytes were of different sizes, the female megaspores tending to be larger, and fewer in number, than the male microspores ...
The life cycle of a gymnosperm involves alternation of generations, with a dominant diploid sporophyte phase, and a reduced haploid gametophyte phase, which is dependent on the sporophytic phase. [3] The term "gymnosperm" is often used in paleobotany to refer to (the paraphyletic group of) all non-angiosperm seed plants.
Cheetahs can go from 0 to 97 km/h (0 to 60 mph) in less than 3 seconds. [109] There are indirect ways to measure how fast a cheetah can run. One case is known of a cheetah that overtook a young male pronghorn. Cheetahs can overtake a running antelope with a 140 m (150 yd) head start.
The entire gametophyte generation, with the sole exception of pollen grains (microgametophytes), is contained within the sporophyte. The life cycle of a dioecious flowering plant (angiosperm), the willow, has been outlined in some detail in an earlier section (A complex life cycle). The life cycle of a gymnosperm is similar.
The female's home range's size can depend on the prey base. Cheetahs in southern African woodlands have ranges as small as 34 km 2 (13 sq mi), while in some parts of Namibia, they can reach 1,500 km 2 (580 sq mi). Female cheetahs can reproduce at 13 to 16 months of age and with a typical age of sexual maturity between 20 and 23 months. [40]
Pteridophyte life cycle Just as with bryophytes and spermatophytes (seed plants), the life cycle of pteridophytes involves alternation of generations . This means that a diploid generation (the sporophyte, which produces spores) is followed by a haploid generation (the gametophyte or prothallus , which produces gametes ).
Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain—either from the stigma (in flowering plants) to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms.
Reproductive spores are generally the result of cell division, most commonly meiosis (e.g. in plant sporophytes). Sporic meiosis is needed to complete the sexual life cycle of the organisms using it. In some cases, sporogenesis occurs via mitosis (e.g. in some fungi and algae). Mitotic sporogenesis is a form of asexual reproduction.