A condition in which an organism produces two different types and sizes of spores, viz. microspores and megaspores. As a reproductive strategy, heterospory involves the following steps: (1) The mature diploid plant (sporophyte) produces "male" and "female" spores. Typically, the plant produces numerous small "male" spores (microspores) and fewer, but larger "female" spores (megaspores). (2) The megaspores are usually retained on the sporophyte, where a haploid female gametophyte develops inside the spore. (3) The microspores may or may not be retained on the sporophyte. They develop into sperm-producing, haploid gametophytes. (4) The sperm are released into the environment, and dispersed between plants. (5) The sperm fertilize female gametophytes grown from megaspores, producing a diploid zygote (fertilized cell). (6) The zygote develops into an embryo and finally a new diploid sporophyte. Heterospory evolved a number of times among different ancient plants. Lycopsids, ferns, sphenopsids and progymnosperms all developed heterospory. Many of these plants also evolved into large trees. The obvious advantage for heterosporous trees is that their great height would enhance the dispersal of windborne microspores. Competition for light in the shady Archaeopteris forests, may have also have encouraged heterospory, since the macrospores, like seeds, included food reserves to assist early plant growth. Heterospory is generally regarded as the precursor to the evolution of seeds. Microspore dispersal and macrospore viability in pteridosperms thus become pollen dispersal and ovule viability in seed-bearing plants. The change in sexual reproduction from moisture-bound gametophytes to seed-bearing sporophytes enabled vascular plants to colonize drier habitats and upland environments, and also provided the embryo with a protective case against hungry invertebrate herbivores.