What Is A Seed?

Although they come in many sizes, all seeds give the same impression - that they're DEAD. Upon catching our first glimpse of a packet's often miserable amount of contents, it is hard to conceive that each seed case contains a well-developed multi-cellular young plant, complete with embryonic roots, stem, with leaves already formed.

Of course, it's not too difficult to understand why this should be. When we think of living things, we tend to compare them to ourselves. And it's hard to imagine surviving for very long after being dried out like parchment, and perhaps frozen down to about -17 deg C for a few months (or even years) in the process.

But seeds can easily survive these conditions. And some seeds have survived - without man's help - for centuries, and even longer. Dr. Linda Berg, in her book 'Introductory Botany', states that:

'the longest-lived seed reliably recorded (by carbon dating), a sacred lotus, Nelumbo, from China, germinated after about 1200 years.'

I have even heard stories of corn seeds germinating after being removed from ancient Egyptian tombs, though I don't know how reliable this is. Nevertheless, it makes us think - in fact, it's absolutely staggering. And it's often made possible by the structure of the seed.

SEED STRUCTURE
The embryonic root (radicle) and shoot (plumule) come attached to one (as in Monocots) or two (as in Dicots) 'seed leaves' (cotyledons) The embryo is surrounded by varying amounts of 'endosperm tissue', and the whole lot is encased in the 'seed coat'(testa). Sometimes (e.g. bean) much of the endosperm has developed into cotyledon, or seed leaf.

TWO BASIC STRUCTURES

If you cut open a pea or bean seed (left), you will find they are comprised mostly of two large nut-like cotyledons; there is little soft endosperm. On a bean seed, it is also possible to see the micropyle. This is a tiny hole through which the pollen tube passed through the ovule to allow fertilision the egg.


On the other hand, a corn seed (right) has little structure, consisting mostly of starchy endosperm; with a small cotyledon.

SOURCE OF FOOD
No living thing can exist without a supply of food to supply energy to fuel basic metabolism, albeit this is greatly reduced during dormancy. Dormant animals have a supply of fat; plants vary in both the nature of the food reserve and in where it is stored. Oils or sugars (mostly starch) might be stored either in the endosperm or in the cotyledons.


Reference: Berg, L (1997) Introductory Botany: Plants, People and the Environment (U.S.A.:Saunders College Publishing)