|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
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
|.||New flowering plant
||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
||Companion cells in the phloem
more efficient translocation (although in all honesty we really don't
understand how the phloem works!)
|.||.|| sieve tube companion
||Ovule (unfertilized seed)
within the megasporophyll (carpel) with two integuments around the megasporangium (nucellus).
Incompatibility mechanisms -
greatly reduced -
(yellow region at right) to 8 nuclei.
||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.
|.||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
(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
|.||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.
|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 some sample questions on this part of
the course? |
If so, click the button.
have now been introduced to the Angiosperms. |
Click the button to look at diversity within this group.