Greenhouses come in many shapes and sizes, with different functions. Some people have small greenhouses in their backyard, or as a lean-to attached to their home, called mini greenhouses, or tomato houses. They want to start off their seeds and seedlings in a protected environment as well as those growing plants that need warmer conditions. Others, wishing to extend their living space, have house extensions in the form of mini-conservatories. Commercial companies usually have larger greenhouses for horticultural purposes, but have more substantial structures for plant displays where the public is admitted. For the same reason, botanic gardens usually have a high proportion of greenhouses with substantial structures