Final answer:
Seed plants are predominantly diploid, with their life cycle featuring an alternation of generations including both haploid gametophytes and diploid sporophytes.
Step-by-step explanation:
Seed plants are typically diploid, meaning their cells contain two sets of chromosomes, one from each parent. In the life cycle of plants, there is an alternation of generations, where both haploid and diploid multicellular organisms are produced. The diploid phase is called the sporophyte, which produces haploid spores through meiosis. These spores grow into the haploid gametophytes that produce gametes. When two gametes fuse during fertilization, they form a diploid zygote which then grows into a new sporophyte, thus completing the cycle. This lifecycle showcases the alternation between the two stages: the haploid gametophyte and the diploid sporophyte.