Introduction
to the
Flowering Plants

 

The flowering plants or angiosperms emerged in the Cretaceous period, some 130 million years ago. Their origin Charles Darwin described as "that abominable mystery".  These sporophytes dominate the landscape around us and are the most successful plant group, with something like a quarter of a million species described. Angiosperm means "contained seeds" and unlike the previous seed plants examined, the ovules are sealed within the carpel and the seeds sealed within a fruit. 

 

People become so obsessed with flowers it is important to remember a flower is nothing more than a cluster of spore-bearing leaves surrounded by whorls of  protective and often albeit attractive leaves. 

The flowering plants are classified into 300+ families, largely on the basis of the flower, their reproductive organ. Imagine recognizing an organism on the basis of just its reproductive parts! (see Schiebinger, 1996

   

The Magnolia flower (right) shows many features of the earliest flowers. (Soursop flowers are a good local equivalent.) Spirally arranged microsporophylls (anthers) and megasporophylls (carpels) are surrounded by non-spore-bearing leaves (petals and sepals which together are termed the perianth). 

The earliest pollinators were just beetles chomping their way through flowers, accidentally spreading pollen and, in view of the bounty of flower parts, sparing some carpels to form seed and fruit. The insects and flowers co-evolved and flower structure and insect mouth parts have come a long way from these rather haphazard initial relationships. 

More on flower evolution...

 

 

. New flowering plant features: 
 
.
 
 
 
 
 
 


Vessels in the xylem
 
 more rapid water transport than via tracheids as       vessels are wider and are stacked one on top of the next forming a continuous pipe 
 
 
tracheids               vessels 
 
 

Companion cells in the phloem

more efficient translocation (although in all honesty we really don't understand how the phloem works!) 
 

. .   sieve tube   companion cell 
 
 
 
 
 

Ovule (unfertilized seed) completely enclosed  
within the megasporophyll (carpel) with two integuments around the megasporangium (nucellus). 
 

Incompatibility mechanisms - 
The pollen tube must grow through the megasporophyll (style portion) to get to the ovule. As a result, a range of incompatibility mechanisms have evolved to control what pollen grain is allowed to germinate on the stigma, what pollen tubes are permitted to grow through the style and ultimately what male gamete will fuse with the egg

 
 
Female gametophyte greatly reduced - 

 (yellow region at right) to 8 nuclei. 
 There is no archegonium.

 
Biodisc photomicrograph
 

Unique double fertilization - 

one male nucleus fuses with the egg to form the zygote which develops into a miniature plant, the embryo. The other male nucleus fuses with two polar nuclei to form a triploid endosperm (orange at right) which develops simply as a food reserve for the developing embryo (red at right). 

Coconut water is actually a liquid endosperm undergoing free nuclear division to feed the young embryo in the coconut fruit! 
 


L.S. Castor oil seed
 
 


Origin of fruit -  
development of the megasporophyll (carpel) and often other tissues to form a special structure for dispersal of seeds. 
The botanist's definition of a fruit often differs from the lay person's. Fruit are not necessarily sweet and juicy. They can be dry and woody. Tomatoes and cucumbers are fruit even if to the average person they are vegetables.

. Sexual reproduction: 
 
.
. Production of microspores and male gametes .
 .  
           A                  B                   C 
 
 
. 
 
 
 
 
In the pollen sac (microsporangium) of the anther, haploid microspores (A) are formed by meiosis. Mitosis then follows to produce a two-celled pollen grain (B) with a small generative cell and a large vegetative cell. This generative cell will undergo further mitosis to form two male gametes (nuclei). The pollen tube grows though a pore in the pollen grain, with the tube (vegetative) nucleus at its tip and the male nuclei behind. 

Pollen grains of different species are quite distinct. Have a look.
Pollen grains were used to test whether the Shroud of Turin really came from Israel.


.
. Production of megaspores and embryosac  .

 
 
 
 
Egg cell surrounded  
by 2 synergid cells  
  

2 polar nuclei 
  

3 antipodal cells 
 
 

.  functional megaspore                             embryosac 
 (other 3 aborted) 
 
.
. In the nucellus (megasporangium) within the ovary, each megasporocyte undergoes meiosis to form 4 haploid megaspores, only 1 of which survives. This divides mitotically to form an
8-celled embryosac.
 
 
.
. Fertilization .
. Once the pollen grain has germinated, the pollen tube grows down through the style and enters the embryosac through the micropyle. The pollen tube apparently homes in on the egg cell based on chemical signals from the synergid cells. The 2 male nuclei are released, one fusing with the egg to form the zygote and embryo, the other fusing with the 2 polar nuclei to form the triploid endosperm, a nutritive tissue. 
 
A wide range of seed dispersal mechanisms have evolved. 
 
..

   

. In summary:
    Like all seed plants, the sporophyte is the dominant generation in the flowering plants, varying widely in vegetative structure. 
The vascular tissue is more specialized with two innovations - companion cells in the phloem and vessels in the xylem.
The complete enclosure of the ovules by the carpels has lead to the evolution of mechanisms which debar certain types of pollen access to the ovules. 
The gametophyte has been internalized (endospory and reduced to 3 cells in the pollen grain and 8 cells within the ovule.
Double fertilization with both male gametes in the pollen tube functional is unique to the Angiosperms.
The further development of the carpel(s) to produce a structure to aid in dispersal of the seed(s) - the fruit - is also unique to Angiosperms.

   

Click left for a handout on the major steps in land plant evolution.
Click left for a handout on equivalent terms in lower and higher plants.

Would you like to look at the lab for this part of the course? 
If so, click the button. 
Would you like to look at some sample questions on this part of the course? 
If so, click the button. 
You have now been introduced to the Angiosperms. 
Click the button to look at diversity within this group. 


   .  

C.M. Sean Carrington, 29 May, 1998
updated August 10, 2004.