The Conquest of the Land
The first plants were algae and these still thrive in a range of aquatic habitats today. (Do not fall
into the trap of thinking because algae have been around for so long they are "primitive". Look
upon them instead as being simple compared to more complex groups. The fact is algae today are
themselves highly evolved and well adapted to the niche they occupy.)
The land plants evolved from the algae, more specifically green algae, as suggested by certain
common biochemical traits;o
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Reserves: starch,
Cell wall: based on cellulose microfibrils,
Photosynthetic pigments: Chlorophylls a & b with accessory pigment ß-carotene
In view of the relatively stable nature of an aquatic environment what would be the selection
pressure favouring algae which could survive on the land? Probably competition!
What kinds of problems would such pioneer plants have faced?
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Desiccation - the plants would dry out,
Water for reproduction - even if there was sufficient water for survival they would need
free water for fusion of gametes,
Support - buoyancy supports and spreads the algal thallus. These plants would now be
plastered on the mud,
Water for spore dispersal - to colonize new terrestrial habitats spores would have to be
released in air not water
The hypothesis is that some 400 million years ago freshwater, green, filamentous algae invaded
the land. These probably had an isomorphic alternation of generations.
gametophyte
sporophyte
Imagine then, within a population of such algae, individuals more able to withstand periods
without submergence. Such individuals which could survive at the pond margins on the wet mud
would be selected.
Gametophytes would need water for reproduction - the basal part of the gametophyte developed
with loss of the upper portion. A sterile jacket of cells evolved to protect the developing gametes
during periods of exposure.
Sporophytes - spore dispersal was originally in water. Spores now need to be dispersed in air.
The upper spore-bearing part of the plant would need to be held above water, this involved
several changes.
The generations began to diverge.
gametophyte
sporophyte
Sporophyte changes
Cuticle - a new non-cellular, waxy, water-proof layer, the cuticle, evolved on the surface of
the entire sporophyte. This minimised desiccation .
Sporopollenin - a similar waxy waterproofing appeared on the surface of spores and
prevented them drying out as they travelled in the air.
Transport system - with the increased bulk of the sporophyte, simple diffusion would no
longer be adequate - evolution of the stele, i.e. xylem and phloem for transporting water,
minerals and organic material.
Ventilation system - this increased bulk also meant gas exchange across the plant surface
and within the sporophyte would have become problematic. The evolution of stomata -pores
in the plant surface - and intercellular air spaces solved this.
Basal portion - the lower portion of the sporophyte attained a role of anchorage. It became
little more than a few thread-like rhizoids.
NOW READ YOUR TEXT PAGES 449-451
ANSWER THE FOLLOWING QUESTIONS:
1. What biochemical traits suggest that land plants evolved from green algae? /3
2. What would be the selection pressure favoring algae which could survive on the
land? This handout gives you one, what is it? Try to hypothesize another on your
own! /3
3. Based on the information in this handout as well as from your text, explain five
problems that plants faced when evolving to live in a terrestrial environment? /5
4. What are the changes that the gametophyte (/2) and sporophyte generation (/5)
underwent in order to be able to withstand periods without submerging? (ie above the
water)
5. Approximalty how many years passed between the evolution of algae that could live
out of the water part of the time, and the first land plants? (/1)
6. What are the two major groups of land plants that evolved? (/2)
7. Based on what you read in 21-1 of your text, would you say that mosses or ferns are
more advanced in their adaptations to a terrestrial environment? Explain your
reasoning! ( /3)
THESE QUESTIONS ARE DUE TOMORROW!!!!
I STILL HAVE NOT RECEIVED MANY OF YOUR REPRODUCTION QUESTIONS.
GET THEM IN ASAP!!!