REPRODUCTION: FINITO!

Endosperm is genetically distinct from the embryo  -- it's triploid.
 Surrounds embryo. Again, it's serves an absorbtive/nutritive function in seed.

As embryo grows, ovule changes.
 Becomes SEED.
Outer layers become specialized as a seed coat.  Impervious, protective.

Embryo undergoes limited development.
 It develops, then stops
     Of what selective advantage is this?

Embryo also has 1 or 2 COTYLEDONS.
 These are storage or absorbtive organs.
  Peas, sunflower, bean, squash, maple have 2 cotyledons.
   They are DICOTS.
  Lily, onion, maize, wheat, rice have just 1.
   They are MONOCOTS.

These are the two classes of flowering plants.
 They are marked by other distinctions too.

However, these distinctions are ARTIFICIAL; the two classes are actually polyphyletic

Fate of cotyledons varies between plants.
 In grain such as maize, the cotyledon is plate like and serves to absorb nutrients from the endosperm.
 In bean, squash and peas, cotyledons are large and fleshy.
  They subsume function of endosperm in nutrient storage.

[What is a corn ‘seed’?  It’s really a 1 seeded fruit!
 Embryo is the ‘germ’.
 Endosperm is what goes into white flour -- consists largely of starch and protein.
 Bran? Tough ovary wall and seed coat.

Let’s return to the ovule....

Remember, as a result of double fertilization, fertilized  egg or zygote is formed.
 Develops into embryo.
Endosperm proliferates as storage or exchange organ.
 In many cases, the cotyledons assume the function of endosperm.
 Outer layers of ovule specialize as impervious seed coat.
       I.e., ovule becomes SEED.

So seed becomes “a kid with a coat and a box lunch”.
 Nothing like this in animals - Again, what kind of organisms are plants, viz their life style?    THINK EVOLUTIONARILY.

This allows the next generation to survive under  adverse environmental conditions, when the  organism is a sessile photoautotroph. Note, it’s a  GROWTH and DEVELOPMENTAL response, not a  behavioral response, to the environment, as are all  plant responses.

Now, how to get the kid 'out the door', or away from the parent plant?!
 Why is that important?

 How is it accomplished?
More dispersal mechanisms, this time for seed.
As embryo develops, hormones produced by embryo  stimulate ovary wall to enlarge and specialize.

Ovary wall can become fleshy, as in peaches, watermelons, cherries, tomatoes, peppers.
Sometimes other flower parts participate.
 Petals -- apple.

Or, ovary wall dries, splits open.
 Often, ovary wall has appendages on it.  Why?
  E.g. Cotton, cocklebur, certain legumes.
 Or, ovary wall may have wings, as in maple samaras.
  Why?

In many species, the seed itself has special structures,
  such as fluff: milkweed, cotton.
These are all mechanisms that allow SEED DISPERSAL.

OK, the seed has been dispersed, but the embryo is  ‘sleeping’. How does it awaken?
Some seeds as old as 10,000 years have been  germinated -- lupines, magnolias.

Various treatments break seed dormancy.
 Cold
 heavy rain
 fire
 light

How are these adaptive from an ecological and  evolutionary standpoint?

Other interesting questions with ecological and developmental implications:

When seed germinates, root is first organ to emerge. Why? In which direction does it grow? Why?

Shoot then emerges.
   In bean, sunflower, squash, the cotyledons, together with the shoot apical meristem and first true leaves,
     are pushed upwards as the stem grows.
   At first, the stem is hook shaped. Why?

   When the hook emerges from the serl, the stem straightens out. How? What signals this response?
   In grasses, something else happens. The shoot grows straight up, but is protected by a sheath-like organ
    called the COLEOPTILE.

  Reminder: in animals, germline (gamete producing tissue) is segregated in embryo and remains as specialized organs
   in adult: testes and ovaries.
 In plants, germline forms at various times, from  somatic cells. i.e., flowers form on branches each year!!!!
  Again, think of this as a crucial adaptation.   Why?

Another important characteristic in plants, as an adaptation to their life style:
 Plants also do VEGETATIVE, or asexual reproduction.
  Runners, bulbs, rhizomes, etc. Remember, even a cut stem or leaf can regenerate roots and become a new plant!!!
 Part of indeterminate growth, regeneration.