Ben Fletcher
The role of stomata in the early evolution of land plants
Few fossils of plants remain from the time they first colonised the land. It is thought that land plants evolved from one ancestor, an alga, in the Ordovician period around 500 million years ago. They split into two main groups. The non vascular plants include mosses, liverworts and hornworts. The vascular plants, which include all other land plants, began to leave many more fossils.
Vascular plants evolved in the Silurian, about 420 million years ago. They have internal supporting and conducting tissues, and usually have stomata in the spore-producing part of the life cycle (sporophyte). This allows them to grow further above the ground and in drier areas than the non-vascular plants.
Some non-vascular plants have stomata and simple conducting tissues in the sporophyte. However, the sporophyte lives out a short life completely dependant on the more conspicuous gamete-producing part of the plant (gametophyte), which is not adapted for life away from water.
The ancestors of the vascular
plants are a group of leafless, simply branching plants with
photosynthetic stems like Cooksonia. These earliest vascular
plants have few stomata on their stems, which is interpreted
as a response to the high carbon dioxide levels in the Devonian,
around 400 million years ago. Although it is not known how the
stomata responded in these early plants to control photosynthesis,
I aim to get a better idea by studying simple plants alive today.
Vascular plants alive today can be divided into two groups, which
diverged in the Devonian.
One group evolved from Devonian plants like Psilophyton.
It contains horsetails and ferns, as well as plants that produce
seeds and are so successful today. Ferns and seed plants developed
wide flat leaves with branched vascular tissues. They are thought
to have developed when tissues filled in the spaces between branching
stems. They provide an obvious advantage in terms of harvesting
light, but it is thought that in the warm, high-carbon dioxide
world of the Devonian, wide flat leaves would have overheated.
In these conditions, plants without leaves like Cooksonia
are thought to have the best shape.
However, carbon dioxide levels dropped towards the end of the Devonian. If these plants responded in the same way as today's plants, they would have increased the number of stomata on their bodies. This would increase water loss through transpiration, which would have cooled the plant. This meant that wide flat leaves would not overheat, and gave the plants that evolved them a great advantage in later times.
The second group, the lycopods,
developed simple leaves supplied by a single strand of vascular
tissue (called microphylls). They are thought to have evolved
from Devonian plants like Zosterophyllum and Asteroxylon.
In the Carboniferous they were a dominant part of the landscape,
with 10m tall lycopod trees like Sigillaria as well as
smaller herbaceous plants. In the Carboniferous, oxygen levels
were higher than today which would have affected photosynthesis
and meant that plants lost more water. They are represented today
by smaller plants such as the clubmoss Selaginella.
I aim to test these theories linking these critical steps in the evolution of plants to changes in the atmosphere. I will be looking at how the stomata of simple plants with old designs respond to recreated ancient atmospheres.
How the atmosphere affects plants
Cooksonia caledonica
