Streamer propagation guided within PTFE tubes were investigated in an 8 cm point-plane gap in white oil. Both positive and negative polarities were studied with a near step  voltage of 50-400 kV. Streamers in a free volume were compared with those in tubes. Within tubes, branching of streamers is suppressed, and interaction between streamers and tube wall may occur. For positive streamers, the guiding tube slightly accelerates streamers and reduces acceleration voltage. However, for negative streamers the tube significantly increases the velocity and drastically decreases the acceleration voltage. Possibly, streamer propagation is governed by the streamers tip-field. When the interaction between streamers and the tube wall appears, the electric field at the streamer tip is increased. This leads to an increase in streamer velocity. The mechanism of streamer propagation is discussed.