Final answer:
The simplest and smallest form of plant life essential to life are microorganisms such as viruses, bacteria, and protozoa. They are present in our environment and play pivotal roles, but some can cause diseases in various life forms. Viruses, while tiny and simple in structure, are significant for their ability to impact human, animal, and plant health.
Step-by-step explanation:
The simplest and smallest forms of plant life are microorganisms like bacteria, viruses, and protozoa. These tiny life forms are present in air, water, and soil, and are essential to life. They facilitate processes like decomposition and nutrient cycling, which are crucial for ecosystem function.
Viruses particularly stand out as some of the smallest biological entities, ranging in size from about 21 millimicrons to 260 millimicrons. In contrast, most bacteria are about 1,000 to 2,000 millimicrons in size. While vital to life, these microorganisms can sometimes cause diseases in plants, animals, and humans.
Historically, humans have been unaware of the existence of these microscopic life forms until the invention of the microscope revealed their presence. Today, they are recognized as fundamental to life on Earth, though some like viruses, challenge our definitions of life since they cannot multiply outside of a living host cell.