Flower structure

Pollination

Angiosperm life cycle

Fruits

Seeds

 

 

 

.*Angiosperm life cycle:

The following diagrams and slides depict the "typical" life cycle.. however variations
exist between species, although the general pattern remains pretty similiar for most species.

 

 

Male gametophyte development: ( 2,3,4,7)

slides modified from> http://virtual.yosemite.cc.ca.us/randerson/Histol.htm

The anther consists of one or more microsporangia, which contain the microsporocytes.

The 2N microsporocyte undergoes meiosis to form 4 1N microspores

 

Each microspore undergoes mitosis to become a 2-celled microgametophyte or pollen grain

The 2 cells include the generative cell and the tube cell

During pollination, the pollen grains are shed from the anther, and hopefully a dispersal vector, whether it be the wind, or an organism lured to the flower or just a casual visitor, carries that grain to the stigma of another flower.

On the stigma, the pollen grain will germinate; this involves the growth of the tube cell down the length of the style to the ovary. This growth may be aided by digestion of the stigma/style or by substances supplied by the carpel. Growth itself is guided via chemotropism, with the release of either hormones or Ca or both by the female.

While the tube cell elongates, the generative cell undergoes mitosis to produce 2 sperm nuclei with associated cytoplasm.

When the tip of the tube intercepts the megagametophyte, it breaks open and the sperm are released, Each is involved in the fertilization process..

 

 

Female gametophyte development ( 4-8 in cycle above)

The megasporangium ( nucellus) forms a megasporocyte which undergoes meiosis to generate (4) 1N megaspores. Of these only 1 survives, the other 3 disintegrating. Note the surviving megaspore to the right....

The survivor undergoes mitosis forming an 2,4 and then a 8 nuclei megagametophyte initially without walls ( embryo sac). Note the 2, 4 nuclei stages to the right.....

3 of the 8 of the nuclei cluster at one end, and during this process of cytokinesis, they develop walls.

At the other end, the nucleus which will be designated as the egg cell along with two synergids are also walled.

 

The remaining 2 nuclei remain in the center enclosed in a singular wall to become the central cell ( nuclei denoted as polar)

The mature gametophyte now consists of 7 cells with 8 nuclei. A far cry from the large independent gametophytes of the earlier plants

Upon fertilization, the 2 sperm enter.

One sperm unites with the 2 central polar nuclei to form the 3N endosperm

the other sperm unites with the egg to form the zygote.

the remaining cells now disintegrate

Mitosis begins in ernest now. The free-nuclear endosperm develops to form the food supply

The embryo eventually forms the leaf cotyledons, a radicle and clearly visible stem region. see seed page for details......

Parthenocarpy: production of fruit without fertilization i.e. pineapple, navel orange, seedless grape.

Auxins, a plant hormone, or synthetically derived growth substances can be applied to encourage parthenocarpy. From the first page, we saw how genetic manipulation can be used to also produce such fruit.