The wide diversity of plants in nature is the result of changes that took place within their life cycle processes. The early water-based plants were limited in their ability to diversify and adapt because of limitations in their physical structures. As changes developed within plant life cycles, new methods for reproduction emerged and enabled land-adapted plants to appear.
According to Ohio State University, plant varieties can be broken down into different subgroups based on physical structure and method of reproduction. Nonvascular and vascular plants differ in their structural make-up, with nonvascular types believed to be predecessors to vascular plants. The term "vascular" refers to a specialised plant tissue that conducts water throughout the plant body. Nonvascular plants lack this material, which is why they're primarily found in water-based environments. With the development of vascular tissue, plants were able to stand upright and take root in soil environments. This change enabled alternate reproductive methods to develop.
Alternation of Generations
The alternation of generations represents the life cycle processes all plants go through, according to Estrella Mountain Community College. It consists of a set of stages in which various plant forms are produced. In effect, the differences apparent throughout the plant species result from how individual plant types move in and out of each stage within the alternation of generations. Life cycles within this process alternate between a sporophyte stage and a gametophyte stage. Sporophyte stages produce spores, whereas gametophyte stages produce male and female sex cells. The more evolved plant types spend more time within the sporophyte stage, while earlier or less evolved plants spend more time within the gametophyte stage.
According to Estrella Mountain Community College, the sporophyte phase comes before the gametophyte stage within the alternation of generations. Spore production occurs through meiosis, a process where plant cell divisions produce male and female spore structures. During the gametophyte stage, gamete cells appear as male sperm materials, or pollen and female egg materials, or ovules. Nonvascular plants spend most of their life cycles within the gametophyte stage as spore structures that produce adult plants. Mosses and liverworts are examples of nonvascular types. Vascular plants, which range from seed-bearing and non-seed-bearing plants to trees and some fern varieties, spend most of their life cycles within the sporophyte stage.
Asexual reproduction in plants involves a cell division process that results in a new version of the original plant, according to Math/Science Nucleus, a science reference site. This form of cell division is known as mitosis. The process starts out with a single cell that splits in half to create two daughter cells. Both daughter cells contain the same genetic material as the original plant, which enables them to grow into individual plant forms. Asexual reproduction can take place in both vascular and nonvascular plant types. Algae are a type of nonvascular plant that uses this method. Vascular types include vegetable plants and some fern varieties.
Sexual reproduction in plants occurs mostly in vascular plant varieties and uses specialised plant structures, according to Math/Science Nucleus. In seed-bearing plants, both male and female structures can exist on the same plant. Fertilisation takes place when pollen particles make contact with the ovule or ovary sac of another plant, or on the same plant. This process is known as pollination and may be helped along by wind currents, insects or animal movements. Once an ovule is fertilised, it produces the seed that will grow into another plant. With non-seed-bearing plants, male and female cone structures reside on leaf surfaces and rely on wind currents to assist in the fertilisation process.