Greenhouses are in far more general use among market gardeners than they were 10 or 15 years ago. They have become especially numerous near railroad lines affording satisfactory shipping facilities and large cities providing good markets for forced vegetables. When a grower learns that a greenhouse is a profitable investment the usual tendency is gradually to increase the area of glass, as many examples of such expansions will prove. The first house is perhaps very small and built for the purpose of starting early vegetable plants, for which it is found convenient and satisfactory ; but the owner is often unwilling to have it idle more than half the year, and, therefore, he tries a forcing crop. If his efforts in the production of crops under glass prove successful, the greenhouse area is increased and new houses are built from year to year, until the grower is known as a vegetable forcer rather than a market gardener. The greenhouses furnish better conditions for starting early plants and they may be used 10 or 11 months in the year if the establishment is properly handled. It is not uncommon for market gardeners to operate an acre or two of greenhouse space, while a much larger proportion of growers have from 1,000 to 10,000 square feet of glass.