The
seed
(text with tooltip)
Unit of sexual reproduction in some plants. Formed when an ovule is fertilized and comprised of outer coat that encloses stored food and an embryo.
habit of most vascular plants almost certainly evolved within a
heterosporous
(text with tooltip)
Heterosporous plants have sporangia that produce spores of different sizes: megaspores (large) and microspores (small). Megaspores produce archegoniate gametophytes, and microspores produce antheridial gametophytes.
line of Progymnospermaophyta. Because the seed, the structure that contained the megagametophyte, was retained on the parent plant, the microgametophyte had to be able to move. In this scenario, the microgametophyte was retained within the
microspore
(text with tooltip)
Microspores is a small spore, and the term is applied to many different types of spores in the microbial eukaryotes and plants. Usually, they are the products of meiosis.
wall and became
pollen
(text with tooltip)
The collective mass of grains produced within the anthers of flowering plants or the male cones of a gymnosperm. In all seed plants, pollen is generated by the development of a microspore into a microgametophyte. The germination of the pollen grain leads to the development of a pollen tube, which delivers two sperm or sperm nuclei to the egg in the ovule. In flowering plants, mature microgametophyte has only two cells, a tube cell and a generative cell.
. The megagametophyte was retained within the sporangium wall and covered by several integuments that were derived from the parent plant. Figure 1 illustrates the salient features of the seed habit with pollen emerging from the microsporangium. The
ovule
(text with tooltip)
An ovule is a structure that contains the megagametophyte in seed plants. The megagametophyte remains within the megasporangium (the nucellus), which is surrounded by layers of integuments. After fertilization, the ovule develops into a seed.
(on the right in Figure 1) illustrates how the integument does not completely close at the apex (the
micropyle
(text with tooltip)
An opening in the integuments of an ovule that exposes part of the megasporangial wall (a chamber called a pollen chamber in gymnosperms). Thus, in gymnosperms, pollen enters the micropyle and germinates in the pollen chamber. However, because the micropyle is not exposed in flowering plants, their pollen germinates on the stigma. The pollen tube grows through the style, and enters the ovule through the micropyle.
) and creates a chamber in which the pollen germinates and sends a
haustorium
(text with tooltip)
Modified hyphae of parasitic fungi that penetrate host cell.
(called a pollen tube) through the megasporangial wall (the
nucellus
(text with tooltip)
Central part of a plant ovule; contains the embryo sac.
) into the megagametophyte where one or more
archegonia
(text with tooltip)
Female reproductive structure that produces and protects the egg in embryophytes. Typically, it has the shape of a cannon with the large egg in the base and a tube of cells, called a neck, extending above the egg. The neck also has a row of cells called the neck canal cells. The ventral neck canal cell lies immediately above the egg.
have differentiated. When the egg cell is fertilized by one of the sperm, the zygote develops into an embryonic plant within the rest of the megagametophyte (now termed the
endosperm
(text with tooltip)
The nutritive storage tissue that grows from the fusion of a sperm cell with polar nuclei in the embryo sac.
). After maturation. the seed has three generations: the integument and nucellus are parental sporophyte, the endosperm is megagametophyte, and the embryo is the daughter sporophyte.
Figure 2 shows illustrations of components of the seed habit from a Paleozoic Pteridosperm called Lygenopteris. Figure 2-A shows the ovule as it would have appeared on the plant. Note that later seed plants have fused the outer petal-like appendages, here called the cupule, into an additional integument. Figure 2-B shows a longitudinal section through the same structure in which the nucellus is surrounded by a single integument that forms a substantial micropyle and
pollen chamber
(text with tooltip)
In gymnosperms, a cavity just above the nucellus in the ovule, the site where pollen accumulates and germinates.
. The pollen-bearing structure (Figure 2-C) shows many microsporangia clustered on a common structure. Each microsporangium bears many pollen grains (Figure 2-D). |
