There are different options to achieve transparency in a game engine.
In the Acknex engine you can achieve this with an alpha channel and with the overlay and d3d_autotransparency (http://www.conitec.net/beta/aentity-overlay.htm) (http://www.conitec.net/beta/ad3d_autotransparency.htm) (http://www.conitec.net/beta/d3d_alpharef.htm)

For drawing the backside of a triangle you set the cullmode within the shader or fixed function to a certain 'value'.

Maybe, these are the most common ways of drawing plants in the different engines - I don't know.

(I am more used to use tgas, as well, but using tgas you need very good sorted triangles within the plant, depends on its complexity, or you need at least a fixed function to skip the sorting)... (Loopix provided that ffp with his plants or with one of his other contributions. Look for loopix-project.com)