Stalked sex organs are found in
Amphibians of plant kingdom
The first vascular plants are pteridophytes. They have two distinct phases in the life cycle as gametophytes and sporophytes. Gametophytes produce haploid gametes. Antheridia and Archegonia are the sex organs, which are multicellular, with sterile jackets but without stalks.
Algae have thalli like body and the gametophytes with sex organs may be same or different. but the sex organs are hidden in the thallus tissue. they do have any stalks.
Bryophytes are the amphibians of the plant kingdom. The sex organs, archegonia and antheridia are produced on the gametophytes, sometimes at the tips of shoots, in the axils of leaves or hidden under thalli. Usually, in bryophytes, the antheridium is borne on an antheridiophore, a stalk-like structure that carries the antheridium at its apex.
Seaweeds are again algae. Seaweed or macroalgae refers to several species of macroscopic, multicellular, marine algae. They reproduce sexually like other fresh water algae.