The effects of plant-activity in the working of nature are significant to all life upon the earth. Plant-activity effects production of plant foods in which simple substances are naturally made into the more complex that alone can nourish the higher forms of physical life, namely, animals and humankind. It effects purification of the atmosphere by removing carbon dioxid from it and returning to it the oxygen from the decomposed carbon dioxid and by taking from the atmosphere some of its free nitrogen through the agency of leguminous plants and transforming this into nitrogen compounds of the soil. It also effects construction of the plant protein from the nitrogen compounds of the soil and carries the mineral salts from the soil into association with organic matter, thus bringing these salts into usable form in human plant-foods. These effects of plant-activity alter favorably the air breathed and construct substances usable as human foods.

Another group of its effects is scarcely less important. As vegetation grows it needs moisture. Where forests have been depleted, the water they would use passes to the streams, that may then overflow, damaging the life they reach instead of serving to increase its security by fertility and an abundant water-supply. Forests modify all wind-effects and break the lower currents of air so that their control is largely determined by whether there are forests standing as a protection to life.

The life-activity of green vegetation in adding oxygen to the air-supply makes life-invigorating the atmosphere of forest regions, particularly those that are evergreen. The currents of air by movement pass some of this fresher air to congested localities. Parks, trees, and gardens in town serve the same purpose there as the forests do in the country at large. Plants in rooms perform a like service during sunlight.