Greenhouses, or as they were known in the 19th century Conservatories, are freestanding structures that house plants and admit light to 'feed' them.
Plants don't move around like animals, so they have to get their food in other ways. To some extent they pull it in from the soil by capillary action. 'Capillary action' happens, for example, when you soak water up with a paper towel held upright. The water flows up against gravity. Roots take in water and nutrients from the soil, while leaves soak up water and sunlight.
Why is sunlight helpful? Because plants use it as an energy source to drive photosynthesis, the process by means of which they use light to power processing and creating nutrients to sustain themselves, grow and reproduce. So, how does a greenhouse help plants any more than just planting them outside in the earth? How do they work?
Greenhouses typically have a roof and walls of glass or translucent plastic. Sunlight consists of a spectrum of light waves, some of which are invisible such as ultraviolet and infrared. Glass, and some kinds of plastic, allow all those waves in but only selected ones get out as efficiently.
Infrared, the wavelength that we sense as heat, doesn't escape back through the glass as readily as the visible wavelengths. That heats the air inside the greenhouse, which is allowed to vent to the outside only partly, as most of them are fairly well sealed.
The combined effect of bringing in all that light energy, while only part of it escapes, causes the temperature to tend to be higher inside than out. Anyone sitting in a closed car in the summer sun is familiar with the effect. That's why greenhouses sometimes used to be called hothouses.
Greenhouses, like a lot of artificial structures, help us control conditions in ways we can't if plants are put in an outdoor garden. The effects of wind, temperature, amount of sunlight and other variables are often harder to control outside a greenhouse.
Along with the ability to regulate the amount of sunlight - by use of glass, shutters or screens - greenhouses make it easier to protect fragile plants from high winds. They also provide an environment that can be more comfortable in which to plant, pot and manage soil.
Along with their utility, greenhouses can be a beautiful addition to the home. Well constructed, or pre-made, greenhouses are often architectural marvels. The glass cage can be lovely, and the view to the variety of plants and vegetables inside turn a workhouse into a work of art.