The present invention relates to a heat retaining agent for agricultural films comprising hydrotalcite-type particles which can satisfy all of a transparency required for heat retaining agents for agricultural films, dispersibility in resins, and handling property at a high level. The valve sleeve is to comprise rigid materialin particular a metal material; the intermediate tube bears clampingly at one end against a hollow shank of the valve body and at the other end surrounds the end region of the valve sleeve, which is in adjacent relationship at a spacing. Furthermore, the invention relates to an individual grain sowing machine with a plurality of sowing assemblies with one aforementioned sowing heart at a time and a corresponding cleaning element. 05-3 times more total light transmittance than when the light is from a direction normal to the incident plane.