The answer is Plants have a two-part life cycle, spending part of their life in a diploid phase and part in a haploid phase.
Let's, for example, take fern life cycle. Their life cycle has alternation of generations - a haploid gametophyte and a diploid sporophyte. Gametophyte produces haploid gametes which fuse to form a diploid zygote. Therefore, gametophyte reproduces with gametes. The zygote grows into diploid sporophyte which produces haploid spores. Sporophyte reproduces with spores, and new gametophyte develops.
