2009 BIO153: Lecture 10
The Vascular Plants
February 11, 2009
Origin of the vascular plants:
▯ current thinking (2006) suggests that the
non-vascular plants (bryophytes) are not a
▯ sister taxon of the vascular plants appears
to be the hornworts
▯ about 50 million years after the appearance
of land plants, fossil evidence of the first
Early vascular plants: a diverse and poorly
understood group. Much of our knowledge of
early vascular plants comes from a large fossil
deposit called the Rhynie Chert (see below).
▯ very simple structure: bifurcating (2-
way branching) stems; no leaves, no
seeds, no flowers
▯ no roots (horizontal stems connected to
soil by root hairs took the function of
roots – anchoring the plant; likely
providing nutrients: note the
difference bewteen these and rhizoids in
▯ sporophyte dominant: sporangia
borne in structures at the top of the
plant – aid in spore dispersal
The Rhynie Chert: a treasure-trove for plant
paleontologists (in this way, analogous to the
1 Burgess Shale for Cambrian animals, which we
will see in an upcoming lecture). The Rhynie
Chert is a silicified moor located about 40 km
from Aberdeen, Scotland. About 400 million
years ago, geysers spouted boiling hot,
siliceous (silica-rich) water, preserving the plant
life so perfectly that the fossils can be
examined at the cellular level. Beautiful!
Fossils from the Rhynie Chert:
▯ vascular tissue with lignin (strong
material that permits greater growth)
▯ stomata & cuticles
▯ sporangia that split (dehisce) – spores
land away from plant (dependent on
moist conditions to complete the life
▯ start to see the first leaves (likely
evolved independently in several
▯ 2 types of leaf structure: microphylls
Recall that we discussed how plant structure
reflects adaptations to living on land. There
were still unsolved problems:
There are distinct competitive advantages to
being taller than your neighbours if you are a
plant. However, it’s hard to grow tall when you
grow only via apical meristem (new cell
growth only at the tips of the plant). Palm
“trees” grow this way: stems thicken but do not
grow outward, making the plant very unstable!
Solution: grow wide as well as tall!
This is accomplished by secondary growth
(outward growth) via vascular cambium.
Today, there are a few vascular seedless plants
that retain many of the features seen in these
early vascular plants. These are:
1. Lycophyta (club mosses)
2. Sphenophyta (horsetails)
3. Psilophyta (whisk ferns)
4. Pteridophyta (ferns)
1. Lycopods - club-mosses ~ 1100 spp.
(**I do not expect you to memorize the
2 numbers of species; only to have a general
idea of which groups are large and diverse,
and which groups are small and relictual).
▯ Lycopods are present today as
small, moss-like plants (you can find
some on the UTM campus!), but in
the Carboniferous, they formed huge
forests of large “trees”.
2. Sphenophytes - horsetails - 15 spp.
▯ all within a single genus
(Equisetum); they are called
scouring rushes due to the high
silica content in their tissues
3. Psilophytes - whisk ferns ~3 spp.
▯ These resemble the Rhyniophyta of
the Rhynie Chert!
4. Pteridophyta - ferns ~10,500 spp.
▯ The largest and most diverse group
of seedless vascular plants. They
typically have large megaphylls, and
many are epiphytic (live on other
Seedless vascular plants dominated the terrestrial
landscape for much of the Palaeozoic, where
conditions were mostly warm and moist, but in the
Permian, conditions especially in the centers of
the continents began to get cold and dry. This
was very problematic for plants that depended on
moist conditions to complete the life cycle, and
many of these forms began to die out. The next
major evolutionary step was the seed.
Advantages of the seed:
▯ protects young plant (sporophyte)
▯ the embryo develops within a seed
coat; supplied by nutritive tissues
▯ seeds can lie dormant and
germinate only when conditions are
▯ the seed is analogous to the cleidoic
egg in animals, which we will
examine in an upcoming lecture.
3 The seed plants are:
1. (Seed ferns)