The first multicelluar plants are believed to have evolved from certain types of single-celled bacteria. These simple cells (called prokaryotes, in contrast to eukaryotes, which have their DNA in a nucleus) share some basic characteristics with plants, including the ability to make their food through photosynthesis. The Endosymbiotic Theory holds that some of the organelles in modern day plants, including the mitochondria and the chloroplast, where photosynthesis takes place, were originally bacteria; thus, a plant cell is likely the result of several bacteria cells having joined together hundreds of millions of years ago.
The first multicellular plants were based in water because they had no specialized way of absorbing water. However, around 450 million years, the first land plants evolved. These plants still lacked the ability to transport water within their different parts, thus they had to be located directly above a water source, but they did have cells walls to prevent drying out. Modern day examples of these plants include mosses and liverworts.
The next important advancement in plants after cell walls was the vascular system, which is a series of connective tissues that allow the plant to transport water throughout its various parts. As a result, plants no longer had to be directly over a water source and they could subsequently spread throughout the surface. Additionally, some plants started to develop seeds, however, they still needed the flow of water in order to transport these seeds. An example of a modern day seedless plant with a vascular system is the fern; an example of a plant with a vascular system and seeds is conifers.
Starting around 100 million years ago, plants developed seed bearing flowers (a plant with flowers is classified as an angiosperm). Flowers have the ability to spread their pollen (which carries the male sex cells) to plants far away. Once the male sex cell fuses with the female sex cell in a flower, it produces a seed. Flowers co-evolved with animals, as it is through animals such as insects and birds that angiosperms commonly spread their pollen.