BIO 131, General Botany
Lecture Notes
Nov. 10, 2008

Vascular plant phyla, continued

Phylum SPHENOPHYTA (or Equisetophyta)    Sphenophytes, Sphenopsids
         weakly heterosporous
         microphylls
         true roots
         photosynthetic gametophytes
         continual for about 300 x 10⁶ yrs
       Equisetum - probably the oldest living group of plants on Earth
                 will see in lab - not necessary to know life cycle
                 weakly heterosporous, true roots
               2 species of Equisetum are native in Kentucky
                          (E. arvense = horsetail; roadside ditches, pond margins)
                          (E. hyemale = scouring rush; stream margins)
        You will see the sporophytes during lab - not necessary to know the life cycle.

Phylum PTEROPHYTA (or Pteridophyta) Ferns
       homosporous or heterosporous
       megaphylls only (none have microphylls)
        photosynthetic gametophytes
        true roots
        continual for about 350 x 10⁶ yrs
        11,000 species alive today, very diverse and widespread
       seed ferns once existed, but became extinct about 150 x 10⁶yrs BP

KNOW THE LIFE CYCLE: typical homosporous fern (pp. 396-397)

HOMOSPORY AND HETEROSPORY

HOMOSPOROUS VASCULAR PLANTS
all meiospores are the same size
all gametophytes are bisexual

HETEROSPOROUS VASCULAR PLANTS
2 different sizes of meiospores
unisexual gametophytes

The 2 different sizes of spores are called--

              MICROSPORES
                microspores are produced in microsporangia, which are on microsporophylls
               produce only male gametophytes

              MEGASPORES
                megaspores are produced in megasporangia, which are on megasporophylls
               produce only female gametophytes

All seed plants (the most successful plants on Earth) are heterosporous.

WHAT IS A SEED PLANT?

When we were looking over the Selaginella life history in class, we noticed how much the young sporophyte germinating from the old megaspore (bottom of p. 386) resembles a seed. Well, there's a reason why they look similar!

A SEED PLANT IS

a heterosporous plant whose megaspores
never are shed from the megasporangium.

Which means that a seed plant female gametophyte develops
inside the megasporangium on a megasporophyll of the parent sporophyte.

(In seed plants, only one of the 4 megaspores in a megasporangium produces
a gametophyte. The other 3 megaspores disentegrate after they are formed from meiosis.)

Here are a few more details. SEED PLANTSAll are heterosporous
All have megaphylls, no microphylls
(and, of course, the sporophytes of all seed plants have vascular tissue)

For seed plants, please note the following:

MEGASPORES
megaspores are never shed from the megasporangium
                WHICH MEANS THAT:
                        female gametophyte develops inside the megasporangium (= nucellus)
                        fertilization occurs inside the megasporangium
                        the young sporophyte (embryo) develops inside the megasporangium
                       (actually only one functional megaspore results from meiosis -- the other 3 disappear)

megasporangium surrounded by integuments = ovule

matured ovule with young sporophyte inside =< /STRONG > seed
integuments of the ovule produce the seed coat of the mature seed

ovules are protected by the megasporophylls
                in Gymnosperms, the ovules are merely protected,
                but in Angiosperms (= Phylum Magnoliophyta or Anthophyta = the flowering plants)
                the ovules are actually enclosed by the megasporophylls

MICROSPORES
      in seed plants,
microspores develop into pollen grains
        each pollen grain contains a male gametophyte
        after pollination, the pollen grain sprouts a tube (called the pollen tube, of course)

the sprouted pollen grain with its pollen tube is the mature male gametophyte

Because of the tube, fertilization in seed plants no longer depends on a film
of water, as it does in ferns and other non-seed plants

This certainly takes some of the chance out of fertilization! - no sperm depending
on a film of water

Because of the pollen tubes, seed plants are sometimes called Siphonogams
(= male gametes in a tube or siphon!)

        In addition, pollen grains are very nutritious (much protein and lipid)
        and a handy size to gather, which means that many animals take an interest
        in pollen - and, as they take an interest in it, they often spread the pollen around
        - we'll get back to that later.

BOTTOM LINE
SEED PLANTS HAVE MORE EFFICIENT REPRODUCTION
   no free-swimming sperm
   protected ovules (off the ground, protected by megasporophylls)
        rich food source is packaged with or inside the embryo, to provide power for germination
                that also attracts animals, and that helps to disperse seeds

We reviewed the life cycles of Lycopodium (pp. 382-383), and went through the life cycles of Selaginella (pp. 386-387), and a typical homosporous fern (pp. 396-397). You should know these life histories.

end of notes for Nov. 10 (some to be repeated Nov. 12)