Lecture 6: Origin of the Land Plants
Algae – not monophyletic, highly polyphyletic. They have arisen through many independent
endosymbiotic events. Multicellularity has evolved 3 different times in alge – red, brown, and green
Green algae are also not monophyletic – they are paraphyletic. This is because land plants arose
from within the green algae. Land plants ARE monophyletic. So, term green algae is not a good
So, land plants are defined by movement onto land.
Problems with no longer living in water?
Dispersal issues – swimming sperm and spore dispersal
Gravity – how do you support yourself?
Pressure – depends on depth
Dessication – physiological, support, photosynthesis, basic biological processes
CO2 – issues about diffusion
Choleachaete and Charales are closest sister groups to land plants. Neither gave rise ot land
What characteristics did land plants inherit from algal ancestors?
Oogamy, matrophy, sporopollenin
Cellulosic cell wall, chlorophyll a & b, phragmoplast, enzymes
Multicellular haplontic lifecycle
Ulvo has altenrationa of generations, some red algae do – these are completely independently
derived. All other algae leading up to land plants are strictly haplontic
Specialized cells, terminal or marginal growth
How did plants solve living on land?
Where to live?
Stay in the boundary layer – they are very small. But this assumes that the surface is being
rewetted. As long as the substrate is wet, humidity will be high, but when it dries, it’s dry.
So, these early plants were small, thin and unprotected from water loss, they absorb water and
CO2 directly from the environment, no true roots, so must be desiccation tolerant.
This means they have an ability to survive cellular dehydration.
Normally, cells cannot go below 99% of relative humidity. But, with desiccation tolerance, they can
go lower. This desiccation tolerance is not really seen in complex plants, mostly just early, small
Desiccation tolerance is largely the domain of the gametophyte, but also found in seeds. Seems to
be incompatible with vascular tissues. In “resurrection” fern, one of few cases where desiccation can occur in diploid stage
What structural adaptions needed to survive (thrive) outside of boundary layer?
Mechanism to prevent water loss from surfaces: cuticle. Waxy layer
Adaptions to control water loss include stomata (turgor controlled valves) and cuticle (water
impermeable layer made of waxes and fatty acids)
Stomata – two adjacent guard cells – oriented bands of cellulose. When stomata are open, they
have the highest turgor pressure. When water becomes limiting, the water pressure decreases and
they relax, so they close.
Stomata: nexus of sensory input and response: light, CO2, water status
Leaf is full of air spaces – so when stomata is open, CO2 can diffuse into air spaces
But, there is a high concentration gradient for water, so open stomata, CO2 rushes in (low