what do u mean by the lines - polyp produce medusae asexually and medusae form the polyp sexually.
Polyp is a sessile form and give rise to medusa by asexual reproduction. By budding polyp divide into medusa which either detach away or remain attached to polyp. Medusa can be visualized as inverted polyp which is free swimming. The structure of polyp and medusa is not much different from each other. Medusa develop either as male or female medusa. Male medusa forms sperm and female medusa forms eggs. The sperm and egg fuse to form larva. This larva grows and settle down to the bottom of water body. Only free swimming form can give rise to gametes that is why polyp can not reproduce asexually.
For better understanding you can visit the following link and see animation regarding it :
There is alternation of generations between haploid gametophyte and diploid sporophyte in the life cycle of a plant. Plants, both haploid and diploid cells can divide by mitosis. Hence, there are two different plant bodies − haploid and diploid. The haploid plant body produces gametes by mitosis and represents a gametophyte. Mitotic division is encountered in diploid cells when zygote divides by mitosis to produce sporophytic plant body after fertilization. This sporophyte produces haploid spores by meiosis. Spores in turn undergo mitosis to form haploid plant body.