Moss is a small, green, non-flowering plant often found growing in dense, velvety mats on the ground, rocks, and trees. These organisms are part of the Bryophyta division and represent some of the earliest land plants on Earth. Despite their simple appearance, moss is fully alive, exhibiting all the biological functions that separate inert matter from self-sustaining organisms. Its active cellular processes confirm its place in the Plant Kingdom.
The Universal Traits of Living Organisms
The differentiation between something living and nonliving rests on a set of universal biological criteria shared by all organisms. A fundamental requirement is organization, which means having a structure based on membrane-bound cells. Living systems must also exhibit metabolism, the process of converting energy and nutrients into the necessary components for survival.
Organisms demonstrate life through growth and development, which is the controlled increase in size and change in form. Another defining trait is reproduction, the ability to generate offspring and ensure the continuation of the species. Furthermore, all living things must possess sensitivity, meaning they can respond to stimuli and maintain homeostasis, a stable internal environment.
How Moss Fulfills the Criteria for Life
Moss fulfills the requirements for life by actively performing complex biological processes at the cellular and organismal levels. Structurally, mosses are multicellular organisms, with individual cells containing organelles like chloroplasts, the sites of energy conversion. Their metabolism is powered by photosynthesis, where they use sunlight, carbon dioxide, and water to create sugars for energy and growth.
Growth occurs through cell division and differentiation, resulting in the main, leafy structure known as the gametophyte. The moss reproductive cycle is a specific type of sexual reproduction called the alternation of generations. The dominant, green gametophyte produces sex cells, and when fertilized, a stalked structure called the sporophyte grows from it. This sporophyte generates and releases microscopic spores, which germinate to start a new moss plant.
Mosses also respond to environmental signals; for instance, their biflagellate sperm must actively swim through a film of water to reach the egg for fertilization.
Why Moss Appears Nonliving
The perception that moss might be nonliving stems from its unique and highly successful survival strategies, which often make it appear inert. Unlike most familiar plants, mosses are non-vascular, meaning they lack the specialized internal tubes (xylem and phloem) to transport water and nutrients efficiently. Instead, they absorb water directly across their surface, like a sponge, leading to a state called poikilohydry. This mechanism means the moss tissue quickly equilibrates with the surrounding atmospheric moisture.
When water is scarce, moss exhibits a phenomenon known as desiccation tolerance, which is the ability to dry out completely to a water content as low as 5–10% of their dry weight. In this desiccated state, the moss enters a metabolically inactive phase called anhydrobiosis, where all life processes cease or slow to undetectable levels. The dried moss looks brown, brittle, and lifeless, but protective molecules stabilize the cell membranes and internal structures. When moisture returns, the moss can rehydrate and resume full metabolic activity, including photosynthesis, often within minutes or just a few hours.