CHAPTER 30 : PLANT REPRODUCTION

Plant Life Cycles:•Mosses•Ferns•Gymnosperms

THE LIFE CYCLE OF MOSSES - Homosporous

THE LIFE CYCLE OF FERNS - Homosporous

THE LIFE CYCLE OF GYMNOSPERMS - Heterosporous

REPRODUCTION IN ANGIOSPERMS

POLLINATION The movement of pollen from the anther (male part) of a

flower to the stigma (female part) of the same or another flower is called pollination. The anther produces thousands of pollen grains, and each one contains a tiny sperm cell that will fertilize the ovum in the ovary of the stigma. Pollen is transferred from flower to flower by small animals, wind, water, and insects .

When pollen lands on the stigma of the same flower, it is called self-pollination. When it lands on a different flower, it is called cross-pollination. However, in order for pollination to actually occur, the two flowers must be members of the same species. Plants have made adaptations for pollination in this way over millions and millions of years to ensure genetic diversity.

Flowers that have both male and female parts are called perfect flowers (those with only male or only female are imperfect flowers).

Examples of pollen types (stained) Pine pollen

FERTILIZATION After pollination occurs, a pollen tube

immediately develops from pollen grain all the way through the pistil, to an opening in the ovule.  Fertilization happens when a sperm cell inside the pollen combines with each ovum to form a zygote.  A zygote is simply a fertilized egg.  Then this develops into an embryo and finally into a seed.

SEEDS AND FRUIT Seeds are made up of several parts: the cotyledon,

endosperm, protective sheaths, and the seed coat are only a few. A typical corn kernel would look like this if you were studying it through a microscope.

The growing shoot and root of the embryo is contained within the protective sheaths. The area of the seed that is stained blue is called endosperm, and it nourishes the embryo as it develops.  The seed coat protects everything inside the seed.  The cotyledon (seed leaf) nourishes the seed after it germinates and as the young plant begins to grow.

DEVELOPMENT OF FRUIT In some plants, many ova may be fertilized in the

same ovary. In others, there is only one egg per ovary. The seeds and the ovary develop at the same time, and the number and size of the eggs fertilized determines how large the fruit will grow. If there are many seeds, the ovary will swell up to a very large size, like in watermelons or pumpkins. If the seeds are small and few in number, the fruit won't be very large, like in apples and cherries.

SEED DISPERSAL

Before they can grow into new plants, seeds need to leave the seed pod. If all the seeds a plant produced landed just underneath the parent plant, they would be too crowded, and the established large plant might not leave them enough light or water for them all to develop properly. The various methods of seed dispersal are designed to ensure that as many seeds as possible have a good chance of growing up to produce seeds of their own.

Sometimes, the pod or fruit containing the seeds is carried away from the parent plant; sometimes, individual seeds are spread to a new location. The size and shape of the seedpod or the seeds influences how they are dispersed. The main methods plants used to disperse their seeds to places with better growing conditions than directly under the parent plant include gravity, animals, force, wind and water. Often, a plant will spread its seeds by a combination of these methods. For instance, a fruit falling to the ground by gravity might then be carried away by animals, or a seed blown by the wind might land in water and be transported somewhere else before germinating.

On some plants native to Australia and South Africa, the seedpods may need the heat of the natural bush fires occurring in these areas to open and release their seeds, which are then dispersed by other means.

http://www.mbgnet.net/bioplants/seed.html

http://www.countrysideinfo.co.uk/seed_dispersl/worksheet.htm