BIL 160 - Alternation of Generations

PLANT REPRODUCTION, GROWTH AND DEVELOPMENT

All members of Kingdom Plantae and some algae undergo a specialized type of life cycle known as Alternation of Generations. This term reflects the fact that the generations change ploidy, with a diploid generation (the sporophyte) giving "birth" to a haploid generation (gametophte), and that haploid generation giving "birth" to the next diploid generation, and so on.

Alternation of Generations: An analogy of the most primitive Alternation of Generations.

If animals were to undergo alternation of generations, then imagine that you are the diploid individual (sporophyte). Your mother, the gametophyte, would be haploid, and would look completely different from you (maybe like a Smurf^TM). Your grandmother would be diploid, and look like you. Your own offspring would look like your mama the Smurf^TM, your grandchildren would look like you, and so on.

Now consider the means by which each generation reproduces.

The haploid Smurf^TM generation has the equivalent of gonads. The female has ARCHEGONIA, analogous to ovaries, and the male has ANTHERIDIA, analogous to testes. Inside each of these sex organs, gametes are produced (ova in the archegonia; sperm in the antheridia) via MITOSIS. (If you're already haploid, you can't undergo meiosis.)

During breeding season, your Papa Smurf^TM would release his sperm to the open world, preferably on a rainy day when they can actually go somewhere by swimming. (Don't try too hard to imagine this.)

Your mother the Smurf^TM would be visited by the swimming gametes of Papa Smurf^TM, which would swarm up her legs, swim into her body, invade her archegonia and fertilize the ova within.

The resulting (diploid) zygote would develop into you. However, poor Mama Smurf^TM is not equipped to give birth. Instead, her embryo keeps growing into a mature individual inside her archegonia, eventually exploding her and smashing her out of existence in the process.
(Aren't you glad animals don't undergo alternation of generations?)
Once you (the diploid sporophyte) have grown to your adult size and are sexually mature, special nodules start to grow in rows along your body. These contain special diploid cells (microspore mother cells if you're male, megaspore mother cells if you're female) which then undergo meiosis to produce huge numbers of haploid spores.
When the spores are mature, the nodules rupture and release the spores to the wind.
If a spore lands in a good, healthy spot, it will germinate and develop into another haploid Smurf (either male or female, with antheridia or archegonia), and the cycle will continue.

Here's a generalized plant life cycle:

And to keep track of this complex cycle, you must learn the following vocabulary...

I. TERMS RELATED TO THE (HAPLOID) GAMETOPHYTE GENERATION:

gametophyte - a haploid plant which produces gametes
gamete - haploid ovum (female) or sperm (male), produced via mitosis in the sex organs of the haploid gametophyte.
archegonium - the multicellular sex organ of the female gametophyte, analogous to an ovary.
antheridium - the multicellular sex organ of the male gametophyte, analogous to a testis.
archegoniophore - a structure which bears archegonia
antheridiophore - a structure which bears antheridia
zygote - single, diploid cell produced by the union of sperm and egg inside the archegonium. This will grow and develop into the sporophyte.

II. TERMS RELATED TO THE (DIPLOID) SPOROPHYTE GENERATION:

sporophyte - a diploid plant which produces spores.
spore - haploid cell produced via meiosis in the sporangium of the sporophyte. It will grow and develop into the haploid gametophyte.
sporangium - structure inside which diploid sporophyte cells undergo meiosis to become spores.
megaspore - a spore which develops into a female gametophyte
microspore - a spore which develops into a male gametophyte
sporophyll - a specialized leaf bearing sporangia on its leaf blade.
megasporophyll - a sporophyll bearing megaspores
microsporophyll - a sporophyll bearing microspores

III. GENERAL TERMS:

dioecious - each individual is either male or female (separate sexes)
monoecious - each individual has both male and female reproductive structures (bisexual)

A Tour of Alternating Generations

First stop, The Bryophytes (as represented by the Liverworts)

Next stop, The Seedless Tracheophytes (as represented by a Fern)

Next, The Gymnosperms (as represented by a Pine)

Last stop, The Angiosperms (as represented by the Lily)

As you can see, the exact appearance and lifespan of the structures listed at the beginning of this lecture varies among taxonomic groups, and there are definite evolutionary trends.

ALTERNATION OF GENERATIONS: A Prose Description of the Anthophyte Life Cycle

Flowering plants have the specialized, spore-bearing leaves we already know as microsporophylls (male) and megasporophylls (female). The sporophylls of anthophytes are arranged into that remarkably diverse structure, the FLOWER, which is a collection of highly specialized leaves (microsporophylls and megasporophylls).

The stalk attaching the flower to the stem is known as the PEDUNCLE, and the slightly inflated "platform" on which the flower itself rests is called the RECEPTACLE. The outermost leaves of a dicot flower are usually small and greenish, and are known as the SEPALS. Collectively, they comprise the CALYX.

Internal to the sepals are the PETALS, often very showy and colorful (though not always!). Collectively, they comprise the COROLLA. The calyx and corolla together make up the PERIANTH. Note that monocot flowers do not have differentiated sepals and petals, and the showy parts of the flower are simply called the perianth (not calyx or corolla).

The male sporophylls, located just inside the petals, are the highly derived STAMENS.
Each stamen consists of a thin, stalklike FILAMENT and the pollen-bearing ANTHER.

Inside the ring of stamens lie one to several megasporophylls. Also highly derived, these are rolled inward to form a structure known as the PISTIL. This consists of a swollen, ovule-containing base known as the OVULARY (or ovary), a stalklike STYLE, and a terminal sticky pad called the STIGMA.

A closer look at the female gametophyte:

Once the ovum is fertilized, the ovule containing it develops into a SEED.

In Anthophytes only, the seeds are borne within a FRUIT:

Which may be one of many different types. (Flower and Fruit morphology are the most important characters a scientist uses when identifying plants! Why do you suppose these structures are so species specific?)

Fleshy Fruits: one or more soft, juicy layer at seed maturity
- berry: all three layers are fleshy
  (examples: blueberry, grape, tomato, orange,
- drupe: exo- and mesocarp are fleshy; endocarp forms a stone
  (examples: peach, cherry, plum, mango, coconut)
- pome: exo- and mesocarp are fleshy (and fused with the sugar-filled receptacle); endocarp forms a papery, five-part center
  (examples: apple, pear, carambola)
Dry Fruits: all layers are hard and dry at seed maturity
- dehiscent (splitting open at maturity to release seeds)
  - legume (unique to the Pea Family)
    (examples: pea pods, bean pods, peanuts)
  - capsule
    (examples: mahogany, anise)
  - silicle
    (examples: many members of the mustard family)
- indehiscent (not splitting at maturity)
  - nut
    (examples: acorn, hazlenut)
  - achene
    (examples: sunflower "seed", the tiny "seeds" on the surface of a strawberry, the tiny fruit on the bottom end of a dandelion tuft)
  - grain
    (examples: fruits of all members of the grass family: wheat, rice, corn, etc.)

What is the significance of the morphologies of these types of fruit, in terms of the natural history (think: seed dispersal!) of the species that bear them?

And from that point, it's up to an array of hormones and other metabolically active substances to take over and guide the development and growth of the new plant.