(horticulture, literally) In a greenhouse, a cold frame, or a similar structure; said of the propagation and growth of plants.
In a glass case, like an item in a museum.
(idiomatic, slang) In jail.
Source: en.wiktionary.orgThe pilasters reaching down were adorned with a glistering substance (I know not what) under glass (as it seemed), resembling a homely fancy, but I judged it to be sugar-candy yet to my raised imagination, divested of its homelier qualities, it appeared a glorified candy. Charles Lamb
The stationmaster's whiskers are of a Victorian bushiness and give the impression of having been grown under glass. P. G. Wodehouse
It is difficult to write about any form of mental disease, especially your own, without sounding as if you were examining a bug under glass. Gene Tierney
Reach inside carefully. Feel my psyche. Make it last. Put this moment under glass. Juliana Hatfield
Friday was the crucifixion Saturday, cremation under glass. The resurrection was on Sunday, no.. correction make it Monday; cause Monday's when they come to take the trash. Tod A