macrospore

A macrospore is a type of spore produced by heterosporous plants, which are plants that produce two distinct types of spores. The macrospore is the larger of these two spore types, the other being the microspore. Macrospores are typically produced in structures called megasporangia, and they develop into the female gametophyte. In seed plants, the megasporangium is called the ovule.

The development of macrospores is a crucial step in the life cycle of heterosporous plants. After meiosis within the megasporangium, usually one of the four haploid cells survives and develops into the macrospore. This macrospore then undergoes mitosis to form the multicellular female gametophyte, which contains the egg cell. Fertilization of the egg cell by a male gamete from a microspore-derived structure leads to the formation of a zygote, which develops into the embryo of a new sporophyte plant.

The distinction between macrospores and microspores is a key evolutionary innovation that contributed to the development of seed plants. This differentiation allows for a more efficient allocation of resources, with the larger macrospore providing sustenance for the developing female gametophyte and eventually the embryo. Examples of plants that produce macrospores include ferns, lycophytes, horsetails, and all seed-bearing plants.