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1. In a greenhouse. Typically used to describe plants grown in a greenhouse. If you want these plants to thrive, you'll need to grow them under glass.
2. In a protective case, especially a glass or otherwise clear one. Come and look at these cool specimens that are under glass. Our pricier jewelry is kept under glass.
See also: glass
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.