Flowering plants (angiosperms) The mature seed is attached to the placenta with seed stalk, externally is surrounded by shells formed from the ovule

Germination of seed of flowering plant and grass

Flowering plants (angiosperms)

The mature seed is attached to the placenta with seed stalk, externally is surrounded by shells formed from the ovule integuments and contains inside the nutrients and the germ (the endosperm, and in some cases also perisperm) (diagram 181).

Among the flowering plants (angiosperms) are distinguished two large groups — monocots and dicots, different number of cotyledons in the seed. Monocots have one cotyledon, dicots have two cotyledons. In addition, they are characterized by a number of other features.

The first phases of embryo development occur equally in both groups. The fertilized ovum divides into two by lateral wall, forming two-cell proembrio. Then in dicots from the end cell develops embryonic stem, cotyledon and gemmule and from apex — germ of root and suspensor. The cotyledons originate in the form of two symmetrical lateral outgrowths. Between them forms the gemmule. Part of the embryo that faces the suspensor forms the root.

During germination of the seed the area, located between the attachment of the cotyledons and root itself, the so-called hypocotyl, extends, lifting the cotyledons above the ground. The lower limit of hypocotyl is determined by the place of appearance of first lateral roots and by the transition of root anatomical structure in the structure, typical of the stem; the so-called root collar. If hypocotyl stays short, the cotyledons remain in the seed shell in the ground. The germ of root is directed to the ground, becoming the main root of the adult plant. Gemmule develops in the stem with leaves (sprout). In monocots from end cell of proembrio develops the germ of the stem, germ of the root and single cotyledon; from its top cell forms suspensor. Soon the embryo takes a little curved shape; its upper part turns into the cotyledons, and on the sides develops hypocotyl and apical gemmule (diagram 182).

Diagram 181. The development of the embryo and of the seed in dicot plant shepherd's purse (Capsella bursa pastoris) (1-7): a — initial germ cell; b — suspensor cell; c — basal cell; d — suspensor; e — cotyledons; f — gemmule; g — the root zone; h — the nucellus; i — integument; j — the antipodes. 8. The structure of the embryo of dicots. 9. The structure of the embryo of monocots; r — the place of the primary root development; gd — a place of gemmule development; hc — hypocotyl; c — cotyledons
Diagram 182. The development of the embryo in monocot plant arrowhead (Sagittaria) (1-7): From the three cells of proembrio (1) develop (2-6) corresponding elements (a, b, c). 7: d — the base of the hypocotyl; e — the initial formation stages of the cotyledon; f — the same — of gemmule; a — suspensor

Grasses

Cotyledon remains within the seed during germination of grasses, closely adjacent to the endosperm as a so-called scutellum. This single cotyledon used for suction of the nutrients from the endosperm that are needed for the development of the germ (diagram 183). Thus scutellum cells highly elongate and penetrate the endosperm. Root (main root) shortly after going into the ground dies and replaces by adventitious roots. Taken from http://worldofschool.org

Diagram 183. Structure and seed germination of grass (wheat): 1 — caryopsis of grass in longitudinal cut: a — the embryo; e — endosperm; al — aleuronic layer. 2. Cut of wheat germ: s — scutellum; end — endosperm; e — peripheral scutellum layer bordering with endosperm; below — this layer cells under high magnification

In some plants, the entire endosperm is consumed by emerging embryo. Their accumulation of nutrients that are needed in the early stages of plant development during germination of the seed usually occurs in the cotyledons. In this case (e.g. beans, peas) cotyledons become fleshy, thick and the embryo is sandwiched between them. These seeds are called protein-free (they do not have “protein” — endosperm). In other plants not entire are endosperm is used by the embryo during its formation, and the remaining part of it is consumed during the germination of the seed. This will be the protein seeds (in grasses).

In monocots, protein-free seeds without endosperm have Alisma, arrowhead (Sagittaria) and other members of the Helobiae order.

Categories: