The plant kingdom is one of the most well-known kingdoms, largely due to its omnipresence in any terrestrial environment. Its maximum diversity is found in tropical environments, but plants also inhabit deserts, poles, freshwater and saltwater environments, and even some live inside animals behaving as parasites. They seem to be only absent in the deep sea or polar ice caps.

The plants group ranges from some microscopic unicellular algae to large trees. Most of their biological diversity, contrary to what one might think, is found in algae, with terrestrial plants simply considered as a group of algae adapted to land. Throughout their evolutionary history, there's a clear trend from mobile unicellular states to multicellular and immobile ones, followed by cellular specialization of works, and finally, with the transition from water to land, the formation of complex body structures, such as the vascular system or flowers.

Plants have very diverse life cycles, involving several generations to complete the cycle. In the lineage that ends up forming terrestrial plants, there's a clear trend from a cycle with a single haploid generation (one copy of each chromosome), to a cycle with two generations dominated by the haploid individual, then to one dominated by the diploid individuals (two copies of each chromosome), and finally to a cycle with one generation dominated by the diploid individual. For much of plant history, they were formed by algae of limited complexity. In the Silurian period (starting 443 million years ago), a small lineage of algae began to adapt to terrestrial life, giving rise to an incredible number of species with highly varied morphologies in a relatively short period of time. These early terrestrial plants had very limited size due to their impossibility in transporting water to all their tissues, but over time, a vascular system was perfected, allowing the development of tree-like forms in groups of plants that are now extinct. With the next geological periods, plants further perfected their adaptation to terrestrial life, giving rise to new groups until flowering plants originated in the Jurassic period. Coevolution between new groups of insects and flowering plants, along with the great adaptive advantages of these plants, has led to flowering plants being the dominant plant group today, with great importance in the ecology of global ecosystems.

The initial organism that ended up forming the plant lineage was not photosynthetic, but rather a unicellular organism that had to ingest food to survive. This primitive organism fed, among other things, on a group of bacteria called Cyanobacteria, the only organisms at that time capable of photosynthesis by breaking down water molecules and releasing oxygen, just like plants do today. There was a close symbiosis relationship between the primitive organism and the photosynthetic bacteria until a moment came when the host, after ingesting a cyanobacterium, did not digest it, so it remained alive inside its cell. With evolution, this relationship of one organism inside another was perfected, giving rise to a photosynthetic eukaryotic organism. In this way, plants originated from the fusion of a eukaryotic organism with a photosynthetic bacterium, which has been reduced to form a structure called a plastid. Plants, like any eukaryote, breathe oxygen, releasing carbon dioxide, but they have metabolic pathways that, with the energy of light, are capable of transforming water and carbon dioxide into sugars, from which they build all the molecules necessary to form their bodies.