A variable Begonia originally described in 1906 and restricted to limestone areas in Sarawak, growing on a vertical cliff. Plants may be plain green, green with white streaks or brown with white streaks as in this case. As far as I know, it has never been brought into cultivation. The plant appears to weather through dry spell very well and has rather tough fibrous stems.
At the bottom of the pic are a couple of Homalomena rubra -an aroid with nice reddish petioles and leaves usually associated with calcerous places.