Objectives:
1) Know the evolutionary steps that produced
the seed.
2) Understand development of an ovule
in a gymnosperm into a seed.
3) Know the main groups of gymnosperms.
Required reading: LIFE pp.595-598. Optional reading is the excellent Chapter 20 in Biology of Plants by Raven, Evert, and Eichhorn (on reserve in the Erindale College Library). Also see Fig. 21.3 of your lab manual.
Study questions:
1) What key aspect of the gametophyte
of early land plants and living seedless vascular plants ties reproductively
these plants to wet habitats or at least to areas that have a significant
wet season?
2) What is an ovule and how does it become
a seed?
3) What is heterospory and why is it an
important step in the evolution of the seed?
4) What is a pollen grain?
5) How many generations are represented
by the tissues in a seed?
6) What is the basic structure of the
male and female cones of a conifer?
Keywords: heterospory, homospory, ovule, microgametophyte, megagametophyte, microspore, megaspore, microsporangium, megasporangium, nucellus, integument, micropyle, pollen tube, pollen grain, pine cone, bract, ovuliferous scale, Cycadophyta, Ginkophyta, Coniferophyta, Gnetophyta.
Tying it all together:
1) Comparisons are often made between
the evolution of the seed and the amniote egg found in all reptiles (including
birds) and primitively for mammals (duck-billed platypus and echidna).
The amniote egg was a significant evolutionary innovation of tetrapods
because it compartmentalized a vertebrate embryo within a series of membranes
that protected and nourished the embryo and enclosed a pool of fluid (amniotic
fluid) that supported the embryo as it developed. The amniote egg
is usually regarded as the means by which tetrapods severed their reproductive
bond to free bodies of water. In what ways is this comparison between
the seed and the amniote egg accurate? In what ways is this comparison
inaccurate?
2) See the Table below. Do the sperm
of seed plants have flagella? Give two reasons why the loss of flagella
is an adaptation for life on dry land.
3) Compare the life cycles of a fern (Fig.
27.22) and a pine (Fig. 27.25). List six differences between these
life cycles.
TABLE COMPARING PHYLA AND SUBPHYLA OF PLANTS - USE THIS AS A MODEL OF
EVOLUTION OF FLOWERING PLANTS - LOOK FOR KEY INNOVATIONS THAT ALLOWED DIVERSIFICATION
AND MOVEMENT INTO NEW HABITATS.
| True vascular Tissue | Altern. of Generations Type II Life Cycle | Oogamy | Gametangia with jacket cells
female archegonium male antheridium embryos nurtured in archegonium |
True leaves | True roots | motile spores | flagellated sperm | Gametophyte | |
| Chlorophyta
green algae |
NO | SOME, isomorphic, homospory | SOME | NO | NO | NO | YES | YES | SOME have,
these are autotrophic |
| Bryophyta
mosses |
NO
but uncertain homology between hydroids and leptoids and xylem and phloem |
YES
N gametophyte dominates homospory |
YES | YES | NO | NO | NO | YES | ALL have, these are autotrophic |
| Rhyniophyta
extinct early vascular plants (includes Rhynia); most primitive known vascular plants |
yes
protostele |
YES | YES | YES | NO | NO | NO | YES | ALL had dichotomously branched stems |
| Zosterophyllophyta
extinct vascular plants that are closely related to club mosses |
YES
protostele |
YES | YES | YES | NO | NO | NO | YES | ALL had dichotomously branched stems |
| Trimerophyta
not monophyletic; extinct vascular plants closely related to ferns, gymnosperms, and horsetails |
YES
protostele |
YES | YES | YES | NO | NO | NO | YES | ALL had |
| Psilophyta
DICHOTOMOUSLY BRANCHED STEMS |
YES
protostele |
YES 2N sporophyte dominates homospory | YES | YES | NO
prophylls |
NO | NO | YES | All have,
HETEROTROPHIC + MYCORRHIZAL SYMBIONT |
| Lycophyta
Club mosses |
YES
protostele |
YES
2N sporophyte dominates some with homospory, some with HETEROSPORY |
YES | YES | YES
microphylls |
YES | NO | YES | All have heterotorphic + mycorrhizal or autotrophic |
| Sphenophyta
horsetails scouring rush |
YES
eustele-like siphonostele |
YES
2N sporophyte dominates homospory |
YES | YES | YES
microphylls through evolutionary reduction |
YES | NO | YES | ALL HAVE, THESE ARE AUTOTROPHIC |
| Pterophyta
ferns |
YES
protostele or siphonostele |
YES
2N sporophyte dominates homospory(some aquatic forms have heterospory) |
YES | YES | YES
megaphylls |
YES | NO | YES | ALL HAVE, THESE ARE AUTOTROPHIC |
| Spermophyta
seed plants |
YES | YES
2N sporophyte dominates HETEROSPORY |
YES | SOME
No antheridia in seed plants No archegonia in angiosperms |
YES
megaphylls |
YES | NO | MOST not flagellated;
sperm of cycads and ginkgos are flagellated |
VERY, very reduced! male is "free" but not photosynthetic, female heterotrophic, parasitic |
Note: All land plants
have gametophytes that can be described as a thallus (i.e. a plant body
that is not differentiated into stem, leaves, and roots). All of
the gametophytes of land plants lack vascular tissue with the possible
exception of mosses depending upon the homologies of the hydroids and leptoids.
Want to learn more?
Images and details of the major taxa of
gymnosperms can be found at:
http://www.geocities.com/~earlecj/taxa.htm
Another web site that covers many of the
plants groups discussed in this lecture:
http://web1.manhattan.edu/fcardill/plants/intro/plantmen.html
*Note that these two web sites place gymnosperms
in a taxonomic group named Pinophyta (either a Division or Phylum).
Based upon your knowledge that gymnosperms are not monophyletic, would
you consider Pinophyta to be a valid group?
Images of the weird and wonderful gnetophyte
Welwitschia
can be found at:
http://www.rrz.uni-hamburg.de/biologie/b_online/d47/welwich.htm