9.3 Reproduction in Angiospermophytes

9.3.1 Draw and label a diagram showing the structure of a dicotyledonous animal

pollinated flower

The four kinds of floral organs are the sepals, petals, stamens (male), and carpals (female).

Their site of attachment to the stem is the receptacle.

• Sepals and petals are nonreproductive organs.

– Sepals, which enclose and protect the floral bud before it opens, are usually green and more leaflike in appearance.

– In many angiosperms, the petals are brightly colored and advertise the flower to insects and other pollinators.

• Stamens and carpels are the male and female reproductive organs, respectively.

– A stamen consists of a stalk (the filament) and a terminal anther within which are pollen sacs.• The pollen sacs produce pollen.• Pollen grains contain sperm cells within them.

– A carpel has an ovary at the base and a slender neck, the style.• At the top of the style is a sticky structure called the

stigma that serves as a landing platform for pollen.• Within the ovary are one or more ovules.• Some flowers have a single carpel, in others, several

carpels are fused into a single structure, producing an ovary with two or more chambers.

• Plant biologists distinguish between complete flowers, those having all four organs, and incomplete flowers, those lacking one or more of the four floral parts.

• A bisexual flower (in older terminology a “perfect flower) is equipped with both stamens and carpals.– All complete and many incomplete flowers are

bisexual.

• A unisexual flower is missing either stamens (therefore, a carpellate flower) or carpels (therefore, a staminate flower).

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• A monoecious plant has staminate and carpellate flowers at separate locations on the same individual plant.

– For example, maize and other corn varieties have ears derived from clusters of carpellate flowers, while the tassels consist of staminate flowers.

• A dioecious species has staminate flowers and carpellate flowers on separate plants.– For example, date palms have carpellate

individuals that produce dates and staminate individuals that produce pollen.

• In angiosperms, the sporophyte is the dominant generation, the plant we see.– Over the course of seed plant evolution,

gametophytes became reduced in size and dependent on their sporophyte parents.• Angiosperm gametophytes consist of only a few

cells.Sporophyte = 2n

Gametophyte = n

Sporophytes make spores (which then turn into gametophytes)

Gametophytes make gametes (sperm and egg cells, which upon fertilization make sporophytes)

Fig. 38.1

• The Flower and its leaves/stems comprise the sporophyte generation.

• The stamens and carpels of flowers contain sporangia, within which the spores and then gametophytes develop.

– The male gametophytes are sperm-producing structures called pollen grains, which form within the pollen sacs of anthers.

– The female gametophytes are egg-producing structures called embryo sacs, which form within the ovules in ovaries.

• The development of angiosperm gametophytes involves meiosisand mitosis.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 38.4

9.3.2 Distinguish between pollination, fertilization, and seed dispersal

Pollination• Pollination is the process that places a

pollen grain (containing sperm) onto the female stigma (sticky end of the style).

• The first step in reproduction

• Flowers attract insects or bees for pollination, or they might rely on wind

Fertilization

• Occurs when the sperm from the pollen grain meets the egg in the ovule

• Occurs in 5 steps:– Pollen grain produces a pollen tube– Pollen tube grows down the style of the carpel– The pollen tube produces sperm– The pollen tube enters the ovary– The sperm moves from the tube to combine with

the egg, forming a diploid zygote.

Before fertilization

After fertilization

Seed Dispersal

• Once the zygote has formed, a seed will develop from it.

• The seed is a protective structure for the embryo

• Seeds may be dispersed by animals (eating fruit and defecating), wind, and/or gravity.

• Once the seed is on the ground and it is in favorable conditions, it will germinate.

9.3.3 Draw and label a diagram showing the external and internal structure of a named dicot seed.

This is a broad bean

The plumule is the shoot apex (embryonic shoot)

The radicle is the root apex (embryonic root)

Seed partsa. Testa protects the plant embryo and the cotyledon

food stores

b. Radicle is the embryonic root

c. Plumule is the embryonic stem

d. Cotyledons contain food store for the seed

e. Micropyle is a hole in the testa ( from pollen tube fertilisation) through which water can enter the seed prior to germination

f. Scar (hilium) is where the ovule was attached to the carpel wall.

9.3.4 Conditions for the germination of a typical seed.

• Seeds require a combination of factors for their successful germination. – oxygen for aerobic respiration – water to rehydrate seed tissues so that they can

become metabolically active– temperature for optimal function of enzymes

• Each seed has its own particular combination of the above three factors.

Some species require certain events to trigger germination:

• Fire

• Freezing

• Passing through the digestive tract of an animal

• Seed coat erosion (poppy seeds)

• Washing to remove inhibitors (beans)

9.3.5 Metabolic processes during germination of a starchy seed.

The metabolic events of seed germination:

a) Water is absorbed and activates the metabolism of cotyledon cells

b) Water absorption triggers synthesis of gibberellin which is a plant growth hormone that triggers the production of the enzyme amylase

c) Amylase converts food reserves (starch) from cotyledons into maltose, which is then made into glucose for energy

d) Some glucose is made into cellulose for cell walls

9.3.6 Control of Flowering • There are many environmental cues that affect

flowering however the photoperiod is the most reliable indicator on 'time' of year (season)

• The photoperiod is the relative lengths of day and night (which changes with the seasons)

• Plants must flower when their pollinators are present, so by using light as an indicator of season, plants can tell what time of year it is and thus when pollinators will be present

9.3.6 Control of flowering in long day and short day plants

• Short day plants (SDP) typically flower in the spring or autumn when the length of day is short. Examples: poinsettieas and asters

• Long day plants (LDP) typically flower during the summer months of longer photoperiod. Examples: lettuce and radishes

Night Length is what really matters

• SDP have a critical long night. That the length of night has to exceed a particular length before there will be flowering.

• LDP have a critical short night. That the length of night must be shorter than a critical length before there will be flowering.

Flowering in SDP:• Short day plants flower when the night period is long.

• In day light or red light, Phytochrome Red (Pr) is converted to Phytochrome Far Red (Pfr). The conversion actually only requires a brief exposure to white or red light.

• In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the conversion.

• Under short day conditions (long night) at the end of the night period the concentration of Pfr is low.

• In SDP, low Pfr concentration is the trigger for flowering.

Flowering in LDP: • Long day plants flower when the night period is

short.

• In day light (white or red) the Pr is converted to Pfr.

• During periods when the day light period is long but critically the dark period is short, Pfr does not have long to breakdown in the dark. Consequently there remains a higher concentration of Pfr.

• In LDP, high Pfr concentration is the trigger to flowering.

Conversion of Pfr to Pr is slow

during the night