TYPES OF PLANTS
Mosses & liverworts
The oldest living category of land plants are the mosses, liverworts, and hornworts. The oldest land plants operate quite differently given that they are 450 million years old. First, they are non-vascular which means they do not have the necessary tissues to transport water and food throughout the plant. They must absorb all water directly through the surface of the plant. This factor contributes to their main characteristic: low growing. It is advantageous to remain low growing because lacking vascular tissue makes it difficult to grow vertically. Secondly, they have an ancient reproductive strategy. The mature plant, sporophyte, produces spores via the gametophyte which the red projections in the top image. When the gametophyte releases spores they can establish anywhere as long as there is moisture.
Mosses
Image by Wikipedia (left)
Liverworts
Image by MSU (right)
FERNS
The first vascular plants appeared about 360 million years ago and are some of the most recognizable plants out there, the ferns. They have a vascular system which allows them to grow vertically and transport water and sugars throughout the individual. These plants are extremely common and have many different species. Similar to the mosses, they also have an ancient reproductive strategy; their sperm has flagella which requires water to move in. This means that reproduction is impossible without precipitation. The sperm must travel throughout water to find another spore that can be fertilized which will then grow into a sporophyte (individual fern).
Gymnosperms
The second to last stop on this tour are the amazing gymnosperms. The first evidence of these types of plants dates to about 320 million years ago. This name is derived from the Latin name for "naked seed," which refers to the cones that these plants bear. The seeds are located on the cone but are completely exposed to the outside elements. Conifers, cycads, ginkos all belong to this class of plants. This is the first evidence of evolution away from flagellated sperm and now reproduction is completely independent from water. The way the gymnosperms reproduce is typically through wind pollination. The trees will produce separate male and female cones, the male cones are yellow and produce tons of pollen while the female cones are the brown pine cones we are more familiar with. The pollen, which is not dependent on water anymore, travels via wind and comes into contact with an ovule located in the cone which then fertilizes and becomes a seed.
Male and Female Cones.
Image by Biology Clermont (left)
Cycad
Image by 123RF (right)
ANGIOSPERMS
Last stop on this tour we have the most recent group of them all, the angiosperms, which came into existence about 125 million years ago. Angiosperm is derived from the latin word for enclosed seed which describes the fruit bearing characteristic of this category; Angiosperms are also known as the flowering plants. Angiosperms are single handedly the most diverse category of plants numbering at approximately 250,000 species. We are much more familiar with their strategy of reproduction which is pollination, which you can learn more about under the pollination tab. They typically rely on another species to transfer their pollen to an ovule offering food source as a reward. However, there are many species that can self-fertilize which can be harmful but species have evolved ways to minimize self fertilization. Their seeds are always enclosed within a fruit hence the name angiosperm
Calochortus tiburonensis
Image by The National Gardening Association (top)
Streptanthus glandulosus secundus
Image by iNaturalist (bottom)
