Endosperm forms a food-storage tissue in the seed. It not only promotes growth and longevity for the seed, but can also facilitate dispersal, as an attractant to animals. Mature endosperm typically consists of tightly-packed cells that contain food-reserve materials such as starch grains or protein bodies. Endosperm is typically a triploid tissue formed by fusion of one sperm cell with two female polar nuclei. It is present in most angiosperm seeds but in greatly contrasting amounts; for example, endosperm formation is negligible in orchid seeds but extensive in grass seeds, in which it forms an important economic crop.
Early endosperm development is traditionally classified into three types, termed nuclear, cellular and helobial, based on the timing and degree of cell wall formation, though transitional forms exist117. Nuclear endosperm is the most common type,and occurs in many eudicots (e.g. Arabidopsis thaliana). Nuclearcell wall formation, and the nuclei are initially free in the cytoplasm of the embryo sac, usually surrounding a central vacuole. Cell walls eventually form in most endosperm tissues, but sometimes the nuclei at the chalazal end remain free; for example, the liquid ‘‘milk’’ of the coconut palm (Cocos nucifera) is a syncy-tium that contains many free endosperm nuclei in addition to oil droplets and protein granules.
In the cellular type of endosperm formation, which occurs in some eudicots (e.g. Acanthaceae), even the earliest nuclear divisions are followed by cell-wall formation. In the helobial endosperm type, which is restricted to some monocots, the primary endosperm nucleus undergoes division to form two unequal chambers, normally a small chalazal chamber and a large micropylar chamber. The nucleus of the micropylar chamber migrates to the top of the embryo sac, and its initial divisions are not accompanied by cell wall formation, though cell walls are formed with later mitoses. The chalazal chamber has far fewer nuclear divisions, and its nuclei remain free in the cytoplasm; it typically has a haustorial role.
Endosperm haustoria may develop in all three types of endo-sperm. Haustoria assist nutrient absorption and sometimes invade adjacent tissues. For example, most species of the mint family Lamiaceae possess both chalazal and micropylar haustoria (Fig. 6.6), which may be either free-nucleate or cellular, some-times even amoeboid. In these species, the first division of the primary endosperm nucleus is longitudinal, followed by formation of a transverse wall. The chalazal nucleus forms a small chalazal haustorium close to the antipodals, and the micropylar nucleus divides further to form a micropylar haustorium and a central cellular endosperm. The micropylar haustorium transfers nutrients from the integument to the embryo and cellular endosperm.
The chalazal haustorium transfers nutrients from the vascular bundle to the endosperm.