Download Lecture 1 Thursday Jan. 4, 2001

BIOLOGY 3404F
EVOLUTION OF PLANTS
Fall 2008
Lecture 20
Tuesday December 2, 2008
Review:
Summary of Plant Adaptations to
Life on Land
In Bryophyta, not Chlorophyta*:
1.
presence of protective layer of cells surrounding the male and
female gametangia,
2. retention of zygote and developing sporophyte within female
gametophyte (= archegonium),
3. presence of a multicellular diploid sporophyte (multiple
meioses per mating event),
4. multicellular sporangia (capsules) with protective layer of
sterile cells,
5. drying-resistant spores with walls containing sporopollenin (a
cyclic alcohol), which is also highly decay resistant.
(*5 sporopollenin is found in zygote walls of Charophyceae)
Onwards and upwards …
These five characteristics are also true of vascular
plants. As we move up the evolutionary tree
towards Anthophyta (Angiosperms), more and
more adaptations to life on land are added.
Protracheophytes
6. Dominant sporophyte (reduces dependence on a
stage that requires free water)
7. Branched sporophyte with multiple sporangia (lots
of spores from one mating event)
Rhyniophyta – 1st true vascular
plants
8. [Here or earlier] Lignin (allows toughening of
secondary cell walls to allow greater height
growth, reduces pathogen attack)
9. A vascular system based on tracheids (allows
plants to grow tall and take advantage of light)
10. [Here or earlier] Development of waxy cuticle
and stomata (keeps water in, allows for entry of
CO2 under controlled conditions)
Progymnosperms - wood
11. Secondary growth: a vascular cambium,
produces wood (secondary xylem) internally and
secondary phloem externally
12. [Here or earlier] Development of the periderm,
which replaces the epidermis in stems and roots
with secondary growth. Consists of cork
cambium, producing phelloderm (living tissue)
internally and cork (= bark) externally. Greatly
protects from drought, including warm days in
winter, and may protect from fire.
[Pteridosperms and]
Gymnosperms
13. Seeds: encapsulation of the daughter embryo in a
protective covering, the integument or ovule wall.
This is like maternal investment (parental care) in
animals - improves the odds of reproduction in
harsh circumstances. There are many associated
adaptations for seed dispersal.
14. Pollen: male gametophyte modified for airdispersal. Motile sperm lost in most groups
except Cycads and Ginkgos.
Gnetophyta (and possibly shared
ancestor with Anthophyta)
15. Vessels: large volume, thick walled (with
spiral or transverse ridges) cells with
connecting pits at their ends, for much more
efficient water transport upwards. Enables
height growth under drier conditions. (Note that
the world’s tallest trees are all gymnosperms
that grow in rain-forest conditions)
Magnoliophyta – flowering plants
16. Carpels: Further protection of the ovule(s)
by enclosing in a modified leaf that
becomes the pistil (ovary, style and stigma).
17. Indirect pollination: pollen lands on
stigma, germinates as pollen tube that
travels down to individual ovule, 2 sperm
nuclei travel to fertilize egg and polar nuclei
Angiosperms II
18. Double fertilization: fertilization of n+n polar nuclei
results in triploid endosperm, the food reserves of
angiosperm seeds (and principal carbohydrate source
for most of the world)
19. Flowers: 4 whorls (sets) of modified leaves that:
a) protect the developing flower (sepals  calyx),
b) attract pollinators (petals  corolla),
c) act as microsporophylls (stamens  androecium),
d) and act as megasporophylls to enclose and produce
the megasporangia (carpels  gynoecium).
and finally …
20. Numerous secondary adaptations to
drought occur throughout the angiosperms e.g., succulent growth forms, tap roots,
CAM photosynthesis, ...
What other MAJOR adaptations to life on
land have been left out of the list? Where
would they fit in the historical perspective?