Beginnings and the Seed part 1

Once fertilization has taken place, the energies of the plant are directed to the successful development of the next generation. The fertilized egg cells are safely protected from the vagaries of climate, or damage by passing insects, by the surrounding mass of the ovary which lies at the base of the now fading flower.

Each part of the ovary has its own role to play in the growth and development of the new seed. Despite a few basic differences, the pattern followed by the growing embryo is characteristic of all plants. At fertilization the egg is made up of a single cell, but this soon begins its growth by dividing into two identical cells. Many more cell divisions follow, but from this point on, however, the newly created cells become differentiated from each other and soon the embryo has three clear parts which will form the above-ground plant, the stem and the roots. The cells which will form the main part of the plant soon show clearly that they are forming into two distinct lobes.

These will develop into the seed leaves or cotyledons and between them a tiny growing point marks the start of all the rest of the above-ground parts of the plant. Plants with two seed leaves are called dicotyledons and include all the broad-leaved species from oak trees to daisies. There is also a group of plants which has only one cotyledon (monocotyledons); these are typified by species with narrow leaves such as lilies and grasses, and in the embryo stage their growing point is set to one side of the single cotyledon. In some plants the first true leaves, as well as the cotyledons, become well developed before germination. The Garden Pea (Pisum sativum) affords a good example of this. At the opposite end of the embryo from the developing cotyledons is a cell which grows much larger than the rest. This eventually divides to form the roots and will be the first cell to grow actively upon germination.

Around the embryo plant is a store of foods known as the endosperm. At first this fills the ovule completely, but as the embryo grows, so it begins to draw on this food supply. In some seeds it completely uses it up leaving the seed with nothing to fall back upon if it is unable to obtain nourishment immediately when it germinates. Most of the very light seeds such as those of poppies and grasses are of this sort. In contrast, the large seeds of the Castor Oil Plant (Ricinus communis) and those of most grain crops have abundant supplies of endosperm, as does the coconut too. It is the endosperm within these seeds which makes them nourishing and desirable to man and animals. A third group of seeds, while containing no endosperm, builds up a reserve of foods in the cotyledons. Most beans and peas are an example of this form of development.

Around the embryo and endosperm, if present, develops the seed wall or testa. Some seeds have also a second layer outside this which serves as extra strengthening, or sometimes a fleshy outer layer. This layer is the pericarp. In the case of many nuts, for example the hazels (Corylus), the outer shell is formed from the pericarp, the actual seed coat being the thin brownish ’skin’ which surrounds the nut. The flesh of a grape (Vitis vinifera) is also formed from the pericarp, as is that of the fruit of Mistletoe (Viscum album), but in the latter the sweet or mucous covering serves as a bait to attract birds which will distribute the seeds. Sometimes more than one layer derives from the pericarp. In the Cherry (Prunus avium), it forms three distinct layers, a very hard inner layer around the kernel — the coat of the cherry stone — a middle fleshy and sweet one and an outer membraneous layer, the skin.

While the embryo is developing, the flower also undergoes changes. In most cases the showy petals soon wither and fall as do the stamens and stigma, though the latter may remain attached to the fruit in a withered state. The calyx, that ring of protective leaves which enclosed the bud, also usually falls, but in a few cases it remains sufficiently firmly attached to the fruit to be a conspicuous part of it. The small brown scale-like remnants at the end of an apple and the larger, often dissected sepals at the end of a rose hip are examples of calyces which do not completely fall. The ovary as it develops becomes the fruit containing within it the seeds developed from the ovule or ovules.

In some plants, it is not only the ovary which makes up the fruit. In both apples (Malus) and roses (Rosa) the ovary is only the central part of the fruit and its outer wall the pithy parts of the core. The rest is made up of the receptacle, that part of the flower upon which the ovary and all the other parts sit. This type of fruit is known technically as a pome.

The number of seeds within the fruit depends upon the number of egg cells which grew in the ovary. Only those which were successfully fertilized can develop. A number of plants produce only one seed to each fruit, the Cherry, the Blackthorn or Sloe Plum (Prunus spinosa) and the Sunflower (Helianthus annuus) being examples. Usually, however, there are two or more and this number can run into thousands; 40,320 have been counted in a capsule of the Corn Poppy (Papaver rhoeas).

The enormous variety of shapes and designs of seeds reflects the many ways in which they are distributed. It is vital for their success in forming new plants that they should be away from the parent plant when they germinate. To grow successfully they need space and not too much competition from their own kind or from surrounding vegetation.

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