| 摘要 |
<p>A film of synthetic plastic material consists of a central band of tansparent or translucent material combined with outer bands of opaque material. The composite films are pref. made by extruding a composite tube having narrow bands of translucent or transparent film and wide bands of opaque film, the tube being cut in the centre of the opaque bands. The tube may be formed by a special extrusion die fed by four separate extruders which are diametrally opposed. By placing the transparent section over the plants their growth is stimulated while the interposed opaque sections retard the growth of the weeds.</p> |
