I went to the botanical garden in the "Jardin des Plantes" in Paris. In a glasshouse with the name "Serre de l'histoire des plantes" on the natural history of plants (exhibiting some ancient members of plants like ferns, Cyatheales, etc.), I found the tree-like plant on the picture attached below (the one with grass-like looking "leaves"; unfortunately, I don't have a better picture because it was a bit hidden). There was no sign labelling this particular plant, or at least I was not able to spot any. Does anyone know what type of plant this is? It was rather small, roughly 2 meters in height.
@sir-thinksalot is correct. This is an Ephedra species. You can have a look at this PDF to get some spatial orientation - it should be with the Gnetales. You can look which species of Ephedra are known to be cultivated in French botanical gardens via a search on the BGCI database. I couldn't find a public database for the plants of the Jardin des Plantes, and it isn't mentioned on the accompanying MNHN website. If you are really interested, you can drop them a line and ask which species it is.