The grass ‘‘seed’’ is actually an indehiscent one-seeded fruit in which the testa and pericarp are fused together to form a caryopsis (Fig. 6.5). The grass caryopsis is an indehiscent fruit (an achene) in which the seed coat has undergone further reduction.
After fertilization the pericarp consists of a few cell layers, and the integuments disintegrate completely, leaving only a hyaline membrane covered with a cuticle, derived from the outer layer of the inner integument. Grass seeds also possess highly diffe-rentiated embryos with a unique highly characteristic prominent outgrowth of the embryo, termed the scutellum, which is normally considered to be a modified cotyledon. Some grasses possess an outgrowth opposite the scutellum, termed the epiblast, which has been variously interpreted as a second cotyledon or an outgrowth of the first cotyledon or of the coleorhiza. Grass embryos are well-differentiated within the seed, prior to germination. They characteristically possess a sheath (coleoptile) surrounding the epicotyl and plumule, and a well-developed radicle also surrounded by a sheath (the coleorhiza). In some grasses (and a few other angiosperms) the outermost layer of the endosperm, termed the aleurone layer, is a specialized tissue of enlarged cells containing protein bodies and large nuclei.