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;-

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?  

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 colonise 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 and were probably heterotrichous.
   

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.

In this model, we would imagine the generations began to diverge.

gametophyte sporophyte

   


But some of these Devonian fossils found suggest both generations were erect

unisexual gametophytes sporophyte

These upright sporophytes, however, seem to be associated with plants whose sporophytes lacked tracheids (like Aglaophyton) or had atypical tracheids (Rhynia).

 

Other Devonian fossils like Cooksonia seem to have had flat, creeping gametophytes

In this photograph, kindly provided by Dr Philippe Gerrienne of the University of Liege, an erect sporophyte of Cooksonia paranensis (labelled S) can be seen near what appears to be a flat, thalloid gametophyte (labelled G).

Plants like Cooksonia (as well as Zosterophyllum) had true tracheids in their sporophytes and so are probably more closely related to the ancestors of the vascular land plants.

For further details see Gerrienne & Gonez (2011).

 


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. 
Multiaxial filament  - this form rather than a flimsy uniaxial filament would provide for an erect plant. Even when the outer cell layers lose water the cells at the core would still be turgid. 
 
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.

 

In the early 1900's, in a quarry in Rhynie, Scotland, fossils were found which very much fit the picture for the sporophyte just described. This plant was given the name Rhynia.
 
 

Rhynia thrived in the Devonian, about 400 million years ago. It had erect dichotomously branched stems with sporangia borne at the tips. It spread by an underground rhizome bearing rhizoids. 
 
The booklet, Plants Invade the Land, describes Rhynia at length.  
 
 
 

Drawing of Rhynia sporophyte by Françoise Gantet

 

Rhynia was superficially very similar to another extinct plant Aglaophyton. Both had stomata and waterproofed spores but the latter did not have true vascular tissue and was bigger.  These fossil plants showed evidence of an alternation of generations.
 
Since then other Rhyniophytes have been described. One of these, Cooksonia, predates Rhynia. It thrived in the Silurian period and its sporophyte is similar in form to Rhynia. These were the first plants to conquer the land. Let's look at these early land plants.

 

When did plants first move to the land?
  • Fossils of isolated land plant spores (Heckman et al, 2001) suggest land plants may have been around as long as 475 million years ago
  • Fossils of land plant spores within plant fragments (Wellman et al., 2003)  date back to 425 million years ago ( these were found in Oman and appear to be liverwort-like, although elaters are noticeably absent!)
  • Molecular clock (i.e.based on DNA sequence evolution) studies predict land plants appeared even earlier - 700 million years ago (Heckman et al, 2001)

 

Interestingly, there is a group of living plants - the whisk ferns - which resembles Rhynia. Psilotum nudum which grows in moist, shady habitats in the Caribbean is such a plant. At one time, Psilotum was thought to be a surviving relative of Rhynia. It is, however, generally thought to be a Fern Ally, related to the Ferns, with loss of features such as leaves and roots.



Gametophyte changes  

Recent views on the early evolution of land plants suggest some modifications may become necessary to the established hypothesis outlined above of flat, creeping gametophytes:-

  1. While Cooksonia seemed to have flat, thalloid gametophytes, fossil Rhyniophyte gametophytes seem to have been erect like their respective sporophytes. They were not low-lying but held the sex organs on erect stalks and even had stomata! (Taylor et al., 2005) 
  2. Some fossil gametophytes of these earliest land plants were unisexual, i.e. either male or female, not bisexual as once predicted (Taylor et al., 2005). This would have promoted out-breeding (and diversity on which natural selection could act).

   

Liverworts may be the closest living relatives to these first  vascular land plants, liverworts and the Charophytes being separate branches from green algal ancestors.

Green Charophytic algae, as exemplified by Chara at right, show some features of land plants. 
 

  • They have a sterile jacket of cells around their gametes. 
  • Mitosis in both groups is similar and quite distinct from that in other algae
  • Both groups show strong similarities in DNA sequence of various genes

   

 
In Barbados, Chara is common in Graeme Hall Swamp. 
 

 

  
Good coverage of this topic is found in the booklet;-

Thomas, B.A. & Cleal, CJ (2000) Invasion of the Land. National Museums & Galleries of Wales.
  

 

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We have now completed our look at the first land plants. 
Click the button to move on to the Bryophytes. 

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© C. M. Sean Carrington 1997
Last modified 7 February, 2013.