Angiosperms, also known as flowering plants, have evolved a fascinating reproductive strategy that allows them to reproduce without the direct need for water during fertilization. Unlike their predecessors, the gymnosperms, which require water for the movement of sperm to the egg, angiosperms have developed mechanisms that facilitate fertilization through other means.
One key reason is the presence of pollen, which is a fine powder containing the male gametes (sperm cells). Pollen grains are adapted to be carried by wind, insects, or other animals, allowing them to travel significant distances to reach the ovules located in the ovaries of flowers. This means that the sperm does not need to swim through water; instead, it can be delivered directly to the female gamete.
Additionally, angiosperms have developed a protective structure called the ovule, which houses the egg. Once the pollen lands on a compatible stigma, it germinates and forms a pollen tube. This tube grows down through the style and into the ovary, where it delivers the sperm cells to the egg, facilitating fertilization without the need for water.
This adaptation not only allows angiosperms to thrive in a variety of terrestrial environments, but it also leads to a greater chance of successful fertilization, as they can take advantage of various pollinators and environmental conditions. In summary, the evolution of pollen and the development of efficient structures for sperm delivery are key factors that have freed angiosperms from their reliance on water for fertilization.