The present invention is directed to a growth medium support plate including an essential planar body part extending therefrom. The growth medium support plate also includes a stem member for connecting a cap to the support plate. The stem member is connected to the body part so as to have a position co-planar with the body part and a position inclined to the plane of the body part. The support plate includes at least one fixed projection extending from and co-planar with said body part adjacently to the stem member and the stem member having in the position coplanar with the body part a releasable connection to the at least one fixed projection.