Section 3: The Basic Parts of a Plant

Glossary

Lesson 7: The Seeds


              
               Some relatively primitive plants, such as mosses and ferns, do not have seeds; they
                use other means to propagate themselves.
               But seeds play a dominant role in plant propagation. The prolific reproduction and
               spread of flowering plants, through seeds, can be readily observed in all areas, from
               forests to grasslands, in both hot and cold climates.




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As you learned in the previous part of this section, a seed is the product of a plant's egg,
that has been fertilized by male reproductive cells, that were transported to the ovule by pollen grains.

But there's more to a seed than that. Every seed has three basic parts:



  • The embryo - every seed is really like a tiny plant, or embryo, with leaves, stems, and root parts. These parts are waiting for the right conditions to make the tiny plant grow, to make the seed germinate.

  • The seed coat - the outer covering of the seed. The coat can be quite thin, or it can be thick and hard. A thin seed coat doesn't protect the embryo very well, and the seed is therefore not very hardy. But thick coats can let the embryo survive through some extreme conditions.

  • The endosperm - a short-term food supply for the embryo. It is used by the embryo to help its growth, after which most of the nutrients are transferred to the cotyledons. The endosperm is contained within the seed coat and is formed at fertilization, but it is not part of the embryo.

  
 There are four steps to the process of seed formation:

   1.   Pollen from the anther lands on the stigma (i.e., pollination). The stigma is sticky so that the pollen adheres to it.

    2.  From each pollen grain, a pollen tube grows down the style and penetrates the ovary.

    3.  The male reproductive cells travel down the pollen tube and joins the ovule (the female egg), thereby fertilizing it.

    4.  The fertilized ovule becomes the seed, and the ovary swells to become the fruit.