Seeds

• SEEDS

  ·       Right after fertilization, a series of changes takes place in the ovule, and, as a result, seeds are formed. Seeds the products of a fertilized ovule in angiosperms, are formed within the ovary.
• A MATURE SEED IS GENERALLY COMPOSED OF:

   

  Embryo

  • baby plant inside the seed

  Seed coat or testa

  • the outermost protective covering that consists of the testa and tegmen. The scar on the seed coat, the hilum, represents the point of attachment to the placenta on the fruit wall.

  Endosperm

  • responsible for the storage of food in the seed. If the endosperm is absent, food is stored in the cotyledons
•  

  DICOTYLEDONOUS SEEDS

  • The seed consists of two cotyledons enclosed in a seed coat.
  • Seed coat:  hilium (scar) and micropyle (small pore)
  • The embryo consists of an axis with its pointed end called the radicle and the leafy end, the  plumule.
  • The seed has two fleshy cotyledons with lot of food material.
  • Example of dicotyledonous seeds are beans, peanut and mangoes
• MONOCOTYLEDONOUS SEEDS

  • Each grain is a small, one-seeded fruit.
  • The seed coat and wall of the fruit are fused together.
  • The endosperm forms the main bulb of the grain and houses the food storage tissues.
  • Its embryo is very small and lies at one end of the endosperm.
  • It consists of only one cotyledon known as the scutellum.
  • The growing shoot tip of the embryonic axis is called the plumule and the root tip the radicle.
  • Monocotyledonous seeds are mostly endospermic.
• SEED DEVELOPMENT

  The fertilized egg cell or ovum grows and gives rise to the embryo, and the definitive nucleus to the endosperm.

  1. The two integuments develop into two seed coats, of which the outer (testa) and the inner one (tegmen.)

  There is usually an outgrowth of the funicle, which grows up around the ovule and more or less completely envelopes the seeds.
• SEED GERMINATION

  • The embryo lies dormant in the seed, but when the latter is supplied with moisture, the embryo becomes active and tends to grow into a small seedling.
  • The process by which the dormant embryo wakes up, grows out of the seed coat, and establishes itself as a seedling is called germination.
•  

  TWO KINDS OF GERMINATION

   

  EPIGEAL GERMINATION

  • when the cotyledons come up above the ground and the embryo resumes its growth.
  • (Ex: santol and mung beans seedlings)

   

  HYPOGEAL GERMINATION

  • the cotyledons remain underground or on the surface. The epicotyl, the portion of the axis lying immediately above the cotyledons, elongates and pushes the plumule upwards.

  (Ex: coconut, corn and mango seedlings
• Many seeds undergo a period of dormancy before germination
• Dormancy is maintained by physiological or mechanical barriers, or both
• In nature, seeds may remain dormant until cracks in the seed coat are created through:

  • Mechanical abrasion by rock particles in the soil
  • Alternate thawing and freezing
  • Bacterial action
• Dormancy may also be maintained by growth-inhibiting substances present in the seed coat
• Inhibitors prevent germination unless there has been sufficient rainfall for a seedling to become established
• Apples, pears, citrus fruits, tomatoes, and other fleshy fruits contain inhibitors that prevent the germination of the seed within the fruits
• Dormin is one hormone that may prevent the germination of seeds
• SEED DISPERSAL

  ·       This is the spread or dissemination of fruits and seeds to distant places. There are several agents that help in fruit and seed dispersal.
• SELF-DISPERSAL

  • fruits of many plants disperse their own seeds by forceful ejection or explosive mechanism.
  • Example of these seeds are acacia and kamantique (Impatiens)
• DISPERSAL BY WIND

  • seeds and fruits have various adaptations which help them to be carried away by wind to short or long distances from the parent plant:
• Some seeds develop one or more appendages in the form of thin, membranous wings
• These wings are light, dry, and usually float in the air, dispersed by the wind
• Examples of seeds with wings include samara and malunggay
• Parachute mechanism is achieved by modified hair-like structures known as pappi (sing. pappus), which open in an umbrella-like fashion
• This mechanism is found in members of the family Compositae
• Censer mechanism is when some fruits, when shaken by the wind, discharge their seeds into the air
• Examples of this mechanism include aristolochia and capa dela reyna
• Some seeds are provided with hairs, either in one or two tufts or all over the body
• Examples of seeds with hairs include talahib, cotton, milkweed, amorseco, and Cyperus seeds
• Papery seeds are very light and paper-like, found in the garlic vine
• Some seeds are very, very light, such as the seeds of some grasses
• DISPERSAL BY ANIMALS

  • many fruits and seeds are provided with hooks, barbs, spines, stiff hairs, and sticky glands on their surfaces, with which they adhere to the body of wooly animals, as well as clothings.

  Seeds of many fleshy fruits are widely distributed by birds. They feed upon the pulpy fruits and drops or excretes them in other locations, where these seeds then germinate.
• DISPERSAL BY WATER

  • seeds and fruits to be dispersed by water usually develop floating devices in the form of a spongy or fibrous outer coat or impervious pericarp, such as in the coconut.
  • Some seeds are just too small and light that they can float on water.
  • (Ex: water lily, coconut, peanut, and many Rhizophora [bakauan] seeds