Fruits and Seeds

click to find the answer to today's question How are "seedless" fruit produced?

grapes are a simple, fleshy fruit Fruit: a matured ovary containing one or more seeds.

In flowers, the ovary is a protective vessel in which ovules are nourished to their mature form, seeds. Within this vessel, the ovules remain attached to parent tissue along zones of placentation. These zones of placentation are known as carpels. Ovaries can be composed of one to numerous carpels. The number of ovules associated with each carpel, and thus the number associated with the ovary, can vary from one to many. Also, some ovaries can be separated into several distinct chambers while others consist of only one chamber. These chambers are called locules. The number of locules is often (but not always) equal to the number of carpels.

Plant fruits are a major portion of the food link to an Internet Websitelink to an Internet Websiteeaten by animals. While you are aware of this, you should remember that not everything defined as a fruit is "sweet".

Basic parts of a fruit: fruit parts of a cherry tomato

  1. Pericarp - the fruit wall (composed of #2, #3, #4).
  2. Ectocarp or Exocarp - the outermost layer of the pericarp.
  3. Mesocarp - the middle layer of the pericarp.
  4. Endocarp - the inner layer of the pericarp.
  5. Placenta - a region of attachment of seeds on the fruit wall.
  6. Funiculus - the stalk attaching the seed to the placenta.
  7. Seed - a matured ovule.

Types of fruits:

Fruit development:

Pollination, and the ensuing fertilization within the ovules, stimulates the ovary enclosing them to begin growing into a fruit. Within a few days after successful pollination the ovary begins to swell perceptible. This is called the fruit set. Fruit development is under the influence of the plant hormones auxin and gibberellin. Pollen is one of the richest natural sources of auxin, so it is thought that the initial stimulus to fruit growth comes simply from the auxin to which the pistil is exposed as a result of pollination. Applying the hormone auxin or gibberellin to the ovaries of flowers can induce a fruit to develop without pollination or fertilization. This is called a parthenocarpic fruit and is, of course, seedless.

When its seeds are ready for dispersal, a fruit ripens. Some characteristics of ripening are familiar: changing from green to some bright color, sweetening, and softening. A fruit's color changes due to chlorophyll breakdown and the formation of other pigments in the chromoplasts. Sweetening is due to the accumulation of sugar, which can reach concentrations of over 20% by weight in some fruits. The softening of fruit tissue during ripening involves the breakdown of cell wall components, particularly pectin.

The ripening can be induced prematurely by exposing fruit to air that contains only a few parts per million of the gaseous plant growth regulator, ethylene. Ripening can be delayed by storing fruits in air kept as free of ethylene as possible and supplemented with extra CO2, which antagonizes the physiological action of ethylene.


Day 3

click to find the answer to today's question Are food reserves inside seeds made of cells?

Seed: a matured ovule. link to a local picture

A typical seed consists of: a typical dicot seed

  1. Embryo - the young plant within the seed consisting of:
    1. Epicotyl - will form the leaf of the new plant.
    2. Hypocotyl - will form the stem of the new plant.
    3. Radicle - will form the root of the new plant.
  2. Endosperm - food reserve derived from fertilized polar nuclei.
    • Cotyledons link to a local picture - serve as food for the sprouting plant.
  3. Seed coat - a structure derived from the wall of the ovule to protect the inner parts.
  4. Hilum - the funicular scar on the seed coat.
  5. Micropyle - a hole through the seed coat.

Embryonic development, occuring within the growing seed, creates a new individual from a fertilized egg. The embryo goes through several increasingly complex stages as its mitotic divisions continue. These stages include the early proembryo, the globular stage, the heart stage (cotyledons begin to emerge), the torpedo stage (differentiation of the vascular tissues begin), and finally the mature embryo.

The endosperm of most developing seeds is at first noncellular or liquid. In both corn and coconuts, a portion of the endosperm remains liquid after the outer part of the endosperm has become cellular. After the endosperm has become cellular, it begins to convert available nutrients into insoluble reserves such as starch, proteins, and vegetable oils, storing up these products for the future use of the embryo during seed germination.

The final important feature of embryo development within the seed is the cessation of growth when the embryo attains full size. This is commonly called embryo dormancy. This is a temporary, physiologically imposed dormancy. If it fails, the embryo continues to grow and the seed germinates within the fruit, the seed becoming useless as a means of reproduction.

fruits are designed for seed dispersal Seed dispersal: spreading the seeds away from the parent plant.

Besides reproducing a species, seeds are also the principal means of migration to colonize new territory or find an environment permitting survival when local conditions change unfavorably.

Dispersal of seeds in nature is accomplished in many ways. Seeds can be dispersed by animals, wind, water, etc. Animal dispersers range from insects to birds; mammals to fish. Modifications in the shape, structure, and often color of the protective ovary directly correspond to the ways in which seeds are dispersed. Bright, fleshy berries are commonly dispersed by fruit-eating birds. Winged fruits, such as those found on maple trees, have obvious modifications to facilitate wind dispersal. link to an Internet Website

In some cases, plant parts such as sepals or bracts that surround the fruit, aid in dispersal. In other cases, seeds themselves bear structures that promote dispersal after the seed is released from the fruit. The term dispersal unit refers to any detached plant part serving as a vehicle for seed dispersal.


Day 4

click to find the answer to today's question How do the cotyledons of monocots and dicots differ as the seedling begins to grow?

Seed germination: the beginning of growth from a seed.

Seed germination is a critical step in a plant's life cycle. Because of the limited size of the embryo and the limited food reserves that can be stored in a typical seed, most seedlings are relatively frail. Yet they must be able to withstand environmental hazards such as storms and temperature extremes, and they must compete with older plants in order to become extablished. Plants have evolved environmental responses in seeds helping to assure that they germinate under the most advantageous environmental conditions, and responses in their seedlings helping them overcome some of the difficulties they often encounter.

Germination begins with the physical uptake of water by the dry seed, called imbibition, followed by resumption of growth by the embryo at temperatures favorable to growth. Some seeds, such as willows, begin to germinate immediately after shedding, as soon as they reach moist, warm soil. Other seeds, such as wild oats, will not germinate when they are shed, but lose their dormancy and become able to germinate after a limited time (usually a few weeks). This is called afterripening. Many seeds possess a more persistent dormancy that can be lifted only by environmental conditions favorable to the success of the seedlings.

Some seeds are dormant merely because they possess tough seed coats impervious to water or oxygen or mechanically preventing growth of the embryo. After dispersal in nature, exposure to the elements gradually breaks down and weakens the seed coats, eventually permitting germination. This behavior tends to space out the different seeds from the same year's crop, depending on the accidents of exposure, allowing the species to take advantage of chance opportunities for seedling establishment whenever they occur.

Other seeds show a self-imposed dormancy of the embryo itself, which can be terminated by a specific environmental signal such as:

Seed viability:

Seeds retain their ability to germinate for greatly varying lengths of time. The seeds of some plants of the moist tropics are viable for only a few weeks while seeds of plants of the dry tropics must possess greater longevity, waiting for favorable conditions. While determining the actual length of the viability of seeds in nature is difficult, controlled experiments have been done.

An experiment begun in 1879 by W. J. Beal of Michigan State University has determined the longevity of weed seeds when buried in the soil. Beal filled 20 bottles with sand holding 50 seeds of each of 20 kinds of nerbaceous plants, mostly weeds - a total of 1000 seeds in each bottle - and buried the bottles 45 cm (18 inches) below the soil surface. Bottles have been dug up at intervals, and germination tests made, ever since. The seeds of only three species survived longer than 50 years. By 1970, after 90 years of burial, only one species, Verbascum blattaria (moth mullein), survived, still giving 20% germination.

Germination of some seeds at much greater ages has been recorded. For example, seeds from a herbarium specimen of the "silk tree", Albizzia julibrissin, collected in 1795 and stored in the British Museum, germinated in 1942, when 147 years old. The record for proved longevity seems to be some viable water lily (Nelumbo nucifera) seeds, dated by the radioisotope method to be about 1040 years old, which were found in a peat layer in Manchuria.

Because of persistent seed dormancy, many of the seeds shed each year do not germinate in that or the next growing season, and some may not germinate for many years, even though still viable. This builds up a "bank" of buried seeds in the soil whose numbers often greatly exceed the number of growing plants in the area. As many as 1000 to 5000 viable seeds per square meter of soil have been found under various kinds of vegetation and 20,000 to 80,000 weed seeds per square meter of cropland. This seed bank enables the regeneration of plant populations after catastrophic events such as fire, flood, and drought. Similarly, weed seed banks in agricultural soils ensure the continual regrowth of weeds despite efforts to control them.

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Parthenocarpic, or seedless fruit can be produced by applying the hormone auxin or gibberellin to the ovaries of flowers.
These hormones can stimulate the ovaries to produce fruit without pollination or fertilization.

 

 

 

 

 

 

 

   

 

At least some of the food reserve in seeds is made of cells.
However, these reserves may also include noncellular substances such as starch and mineral oil.

 

 

 

 

 

 

 

   

 

As a seedling begins to grow, the cotyledons of monocots stay in the soil while those of dicots are pushed above ground.
Much of the monocot cotyledon is made of non-cellular material. Much of the cotyledon of a dicot is cellulear, actually becoming the first leaf of the sprout.