plant with translucent stem

I have just seen a plant in an "Open Garden"that I would like in my garden but the owner does not know its name. It has a singlebtranslucent stem (a bit like a balsam plant but it is a perrenial,not an annual) and has pale greeny- grey leaves with a serrated edge. It is about 6 feet tall and in a pot.She says it has survived the winter but that she put it in her conservatory during the recent freezing spell. Can anyone tell me what it is. please?
Patricia Thorpe

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