Flowering Plant Diversity

   

 Some of the earliest flowers were very simple - just collections of carpels and stamens with no perianth. In fact, the perianth was often just a continuation of the leafy axis and not distinguishable into sepals and petals. Such flowers had numerous stamens and carpels which were free, i.e. not fused to each other. Early flowers were wind- or insect-pollinated, beetles being the earliest pollinators. As mentioned before, the insects and flowering plants co-evolved. Click here to see flowers with insect eyes.

     

The quarter million flowering plants known have been classified into 300+ families. Study of these living plants and fossil angiosperms has led to the following consensus about flower evolution:- 

     

Flower evolution  .
Primitive traits 
  • Flower parts many
  • Flower parts of an indefinite number
  • Flower parts free
  • Flower parts spirally arranged
  • All types of flower parts present
  • Flowers bisexual
  • Ovary superior
  • Flower of regular symmetry
  • Flowers conspicuous

For examples look at the related lab

Advanced traits 
  • Flower parts few
  • Flower parts of a definite number
  • Flower parts fused
  • Flower parts whorled
  • Some types of flower parts missing
  • Flowers unisexual
  • Ovary inferior
  • Flower of irregular symmetry
  • Flowers inconspicuous

     

Flowers vary tremendously in form and size:-  .
 
Lemna (duckweed) is a tiny aquatic plant, common in the Caribbean, resembling a floating leaf, 2-3 mm long. Its inflorescence is minute, one of the smallest known.
 
Rafflesia is a parasitic flowering plant from Asia. Its flower is almost 1 m in diameter and the largest known. These giant blooms may well have given rise to far-fetched tales of man-eating plants! 
 Check out another monstrosity!

       

The Monocotyledons & Dicotyledons 

Very early in the evolution of the flowering plants there was a split into two major groups - the monocotyledonous plants (monocots) and dicotyledonous plants (dicots). Most angiosperms are dicotyledonous. There are several characteristics which separate these two groups. 
 

     

Monocotyledon Dicotyledon
One cotyledon (seed leaf) Two cotyledons (seed leaves)
 
                            Leaves parallel-veined
 
Leaves with reticulate (net) venation 
Scattered vascular bundles Vascular bundles in a ring
 
Flower parts in 3's or multiples of 3
 
Flower parts not in multiples of 3
No true secondary growth - generally herbs Secondary growth - trees
Adventitious roots Primary and adventitious roots

         

Getting up into the light - 
adaptive radiation of the plant body
Vegetation is stratified and the plant form and stature is termed its habit. The following body plans are recognized;- 

herb     non-woody, small plant 
shrub   woody, multi-trunked plant 
tree      woody, single-trunked plant 
vine      twining or climbing plant

There are several variations on this. 

     

Lianas woody vines of tropical forests - rooted in the ground, spreading over the canopy. 
Epiphytes - plants rooted on other plants (but not parasitizing these), simply using the other plant as a foot hold to get into the light. 

e.g. members of the pineapple (Bromeliad), orchid, 
       and anthurium (Aroid) families

       

Arborescent (tree-like) and giant herb monocotyledons - since monocots have no secondary growth none of them are true trees but some have found their own unique way to attain height. 
 
.
 

Giant herbs - bananas, some gingers and heliconias grow to a considerable height without woodiness or even a true stem. What appears to be the trunk is actually the leaf bases, wrapped one around the next. The banana "tree" is one such giant herb. 

banan.jpg (27066 bytes)
 

Palms - just below the apical mersitem, a second (primary thickening) meristem develops which broadens the growing point. This very broad stem then simply elongates, with vascular bundles and fibres at the periphery lending strength to the palm trunk.

Arborescent monocots - some woody monocots (e.g. species of Yucca, Dracaena, Pandanus, Agave) have secondary growth but quite different to that found in dicots. An outer ring of dividing cells produces on the inside new parenchyma with scattered vascular bundles. 
 
 
                          Aloe pillansii in South Africa
bamboos - are woody grasses which attain, in some cases, tree-like stature. Their strength is due to the large numbers of vascular bundles and fibres in the stem periphery, the central pith eventually dying. Bamboos grow extremely rapidly but reach a finite height (determinate growth). New shoots then emerge from the underground stem (rhizome). 

       

Adaptations to extreme environments  
have evolved in a wide range of flowering plant families. 
 

     

 Hydrophytes are "water plants". Flowering plants have re-invaded the water habitat of their ancestors. Some simply float on water, others are partially submerged while others are totally submerged. 
 
Xerophytes are plants adapted to harsh, dry environments. They often have thick cuticles, reduced surface/volume ratios and shallow roots, store water in their leaves or stems and open their stomata only at night. Several plant families are xerophytic, the cacti exclusively so.
..
Pistia stratiotes (water lettuce) is a floating aquatic common in the Caribbean. It is actually in the same family as Anthurium.   
 
Cacti (left) and desert euphorbs (right) are a good case of convergent evolution. The cacti are found in the deserts of the Americas, the euphorbs in the deserts of Africa. Both have green leafless, spiny stems. They can be distinguished by their flowers and by the presence of white latex in members of the Euphorbiaceae. 
 
Victoria amazonica, native to Guyana has floating leaves up to 2 m across. It is the queen of water lilies! 
 
Lithops spp.- living stones- live in deserts in Southern Africa and are extremely well camouflaged against herbivores. 
Halophytes are plants adapted to saline conditions as found near the seashore. Such plants are often fleshy and succulent (to dilute the salt they invariably take up) and often have salt glands on the leaf surface to pump salt out of the plant. Since water availability is a problem they share several features with xerophytes.
Sea grasses are monocots which live in the sea. They form sea grass beds in shallow sandy areas which are important in marine food chains and in stabilizing the shoreline. Other submerged hydrophytes occur in fresh water.  
Fleshy-stemmed Ipomoea pes-capre (seaside yam, patat b-lanm) has salt glands on its leaves. 

     

Alternate modes of nutrition  
have evolved in  a wide range of flowering plant families. 
 

     

 Insectivorous (carnivorous) plants photosynthesize but live in nutrient deficient soils and obtain nitrogen and other nutrients by digesting insects. This life style has evolved 
in 5 angiosperm families. 
 
.
.. Sundews and venus fly traps have leaves covered in glandular hairs which trap & digest the insect. Pitcher plants have vase-like leaves which attract insects by their smell and from which they cannot escape. 
 More here on stinking flowers!
Parasites no longer photosynthesize but invade the body of a host plant and simply tap into its vascular tissue. 

 

Cuscuta (dodder, love vine) is a common pest in the Caribbean. It only photosynthesizes as a seedling. Peg-like haustoria link the vascular tissues of host and parasite.
  No, this is not a fungus but the inflorescences of a parasitic flowering plant, Helosis (photographed here in a St Lucian rainforest). The plant is non-photosynthetic with most of it underground, parasiting forest trees. These structures burst into flower at night!    
Semi-parasites still photosynthesize but supplement their carbon supply by tapping into the body of a host plant. Note true epiphytes are in no way parasitic. 
 
 
 
Mistletoes are the best known plants with this mode of nutrition, parasitizing trees. They are famous in Northern countries as a Christmas decoration. 

     

Click here if you would like to learn more about the flowering plants of the Caribbean.

     

. In summary:
 
 
Several aspects of flower structure are recognized as primitive or advanced, e.g. number and fusion of the flower parts, completeness, protection of the ovary, symmetry. 
Flowering plants are based on two basic plans - the monocotyledons and the dicotyledons - with distinct differences.
Flowering plants vary in habit. The only true trees are all dicots but monocots achieve tree-like stature by several means. 
Flowering plants have invaded a range of extreme habitats by adaptive radiation, e.g. hydrophytes, halophytes, xerophytes.
Flowering plants have evolved alternative modes of nutrition as evidenced by parasites, semi-parasites and insectivorous plants.

    Congratulations on reaching the end of this course.  I hope you enjoyed it and learnt something!    

Would you like to look at the lab for this part of the course? 
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Would you like to look at some sample questions on this part of the course? 
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       .

Sean Carrington, September 25, 2001
last updated March 31, 2005