Seed structure

The basic seed structure is shown in Figure 7.7. The main features of the seed are:

  • embryo, in order to survive the seed must contain a small immature plant protected by a seed coat;
  • testa, the seed coat, is formed from the outer layers of the ovule after fertilization;
  • micropyle, a weakness in the testa, marks the point of entry of the pollen tube prior to fertilization;
  • hilum, this is the point of attachment to the fruit.

The embryo consists of a radicle, which will develop into the primary root of the seedling, and a plumule, which develops into the shoot system, the two being joined by a region called the hypocotyl. A single seed leaf (cotyledon) will be found in monocotyledons, while two are present as part of the embryo of dicotyledons. The cotyledons may occupy a large part of the seed, e.g. in beans, to act as the food store for the embryo.

Seed coat (testa)

Hilum Plumule Hypocotyl Radicle

Cotyledon

Figure 7.7 The structure of the seed, (a) runner bean seed just beginning to germinate and showing developing radicle showing a geotropic response (see page 000), (b) long section of bean seed showing structure

In some species, e.g. grasses and Ricinus (castor oil plant), the food of the seed is found in a different tissue from the cotyledons. This tissue is called endosperm and is derived from the fusion of extra cell nuclei, at the same time as fertilization. Plant food is usually stored as the carbohydrate, starch, formed from sugars as the seed matures,

Goosegrass Seed Dispersal
Figure 7.6 Seeds: a range of species, from top -runner bean, left to right - leek, artichoke, tomato, lettuce, Brussels sprout, cucumber, carrot, beetroot
Bean Seed Testa Micropyle Hilum

Seed coat (testa)

Hilum Plumule Hypocotyl Radicle

Cotyledon

Figure 7.7 The structure of the seed, (a) runner bean seed just beginning to germinate and showing developing radicle showing a geotropic response (see page 000), (b) long section of bean seed showing structure

The seed is formed from the ovule of the flower and is the result of the reproductive process.

e.g. in peas and beans. Other seeds, such as sunflowers, contain high proportions of fats and oils, and proteins are often present in varying proportions. The seed is also a rich store of nutrients that it requires when a seedling, such as phosphate (see p367).

The seed structure may be specialized for wind dispersal, e.g. members of the Asteraceae family, including groundsel, dandelion and thistle, which have parachutes, as does Clematis (Ranunculaceae). Many woody species such as lime (Tilia), ash (Fraxinus), and sycamore (Acer) produce winged fruit. Other seed-pods are explosive, e.g. balsam and hairy bittercress. Organisms such as birds and mammals distribute hooked fruits such as goosegrass and burdock, succulent types (e.g. tomato, blackberry, elderberry), or those that are filled with protein (e.g. dock). Dispersal mechanisms are summarized in Table 7.1.

Seeds are contained within fruits which provide a means of protection and, often, dispersal.

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Responses

  • aurelia
    What structure of a seed develops into a shoot?
    10 months ago

Post a comment