Lifecycle & Ecology
Angiosperms are considered to be one of the greatest examples of symbionts in nature and due to their many mutualistic relationships with pollinators, fungi, herbivores and others. They can be found in almost any environment, so long as there is sunlight, some form of water, and a way to spread their offspring. There are four major events that make up flowering plant reproduction; pollen development, egg development, pollination and fertilization. Angiosperms produce two types of spores; microspores which lead to the generation of pollen and megaspores which form the structure that houses female gametophytes (Boundless, 2014).
Pollen develops inside the stamen. Inside the anther of an angiosperm lie the diploid microspores. These microspores undergo meiosis to become haploid microspores. Haploid microspores then undergo mitosis to develop into pollen grains. Pollen forms from the male gametophyte in flowering plants. The gametophyte consists of two cell types: tube cells to aid in fertilization and generative cells which generate sperm cells (Taiz, 2006).
Egg development occurs inside the carpel. Inside of the ovaries are egg producing structures known as ovules. Inside of an ovule are diploid cells. Similar to the production of pollen, these diploid cells divide via meiosis to become haploid cells that are the megaspores. These spores then go through three rounds of mitosis forming seven cells. One of the cells has two nuclei, called the endosperm and another of the cells becomes the egg. The remaining five cells are not used in reproduction (Boundless, 2014).
Once both eggs and pollen development have taken place they are ready for pollination. Pollination can occur in many ways; two major forms are wind and water dispersal. Unique to angiosperms is the use of pollinators such as birds and bees. The last step in flowering plant reproduction is fertilization. For fertilization to occur the tube cell of the male gametophyte creates a tube to the ovule (Taiz, 2006). The generative cell uses this tube to send sperm down to the ovule so fertilization can occur. One sperm will fuse with the egg forming a zygote the other fuses with the endosperm forming a triploid endosperm cell. This process, known as double fertilization, is unique to angiosperms (Derksen, 2013). The endosperm later develops into nutrient tissue while the zygote divides by mitosis, developing into an embryo which grows into a mature plant. Angiosperms spend most of there life in the adult stage known as a sporophyte. When we see trees, grass, flowers, vegetables in a garden we are seeing sporophytes! Angiosperms are very important due to their abundance and impact on almost every habitat on earth. Due to their diverse morphology they can range from the small to massive, aquatic to mountainous, grass to trees and everything in between.
Evolutionary History
The Amorphophallus titanum, or Corpse flower is one of the most bizarre and oldest ancestors of modern angiosperms. Photo taken by Nick White
Not surprisingly, angiosperms are the most commonly found type of land plant. Angiosperms evolved in the Cretaceous era, around the same time as many groups of modern insects. Many of these insects acted as pollinators that drove the evolution of both angiosperms and the insects themselves. (Soltis 2005). Due to the availability of pollinators (insects occupy nearly every environment on the planet) it has allowed angiosperms to become the most numerous plant found on land. This relationship is considered one of the greatest examples of symbioses in nature due to their many mutualistic relationship with pollinators, fungi, herbivores and others. They can be found in almost any environment, so long as there is sunlight, some form of water, and a way to spread their offspring.With co-evolution, these two species have been able to occupy places that few other species previously could, changing the habitats of the entire planet.(Lerner 2008)
Unlike many land plants, angiosperms did not evolve from gymnosperms. It is unclear what type of plants gave rise to angiosperms. (Angiosperms 2014) Some scientists believe that a group of plants known as “seed ferns” ,or pteridosperms, may have been the progenitor of the angiosperms. These “seed ferns” were around for many millions of years before angiosperms and yet have similar traits like seed-bearing capsules and specialized organs that produced pollen. While we are still not exactly sure how ancient angiosperms may have come about, we have an idea of what these ancestors may have looked like. They were likely small with small flowers. The flowers were probably green and not at all like the flowers we are used to as their sepals and petals would not be separated or distinguishable (Angiosperms 2014). And while the exact way that angiosperms evolved to what we know today is still unclear, their impact on our world today is obvious.