The fiber content of leaves and stems varies, tending to increase with age. Seeds, fruits and tubers come in variety shapes, sizes and colors. A seeds has an outer coat to protect the embryo as the seed lies dormant during the winter. 
Come the warm days of spring and the seed begins to germinate, pushing a new shoot through the now cracked outer coat to reach the soil surface and point its tiny leaves towards the sun. In the period between the onset of germination and the emergency of the seedling into the sunlight, the energy reserves of the seed are used up in growth. Some seeds (e.g., soya) use oil for these energy reserves, but most (cereals, rice, cassava, potatoes) use starch.
Although there is some fiber in these starchy stores it is present in much greater quantity in the structural part of the seed – its protective coats and its plant embryo, or ‘germ’. It is these coats and germs which man chooses to remove and discard, so that his enjoyment of the starch core is unimpaired. That is a botanical description of fiber.
There are four main molecular species which make up fiber: cellulose, hemicellulose, pectin and lignin. The first three can be classified as carbohydrate lignin being unclassified in the nutritional sense. Cellulose, for example, is comprised of long chains of glucose molecules joined end to end. Cellulose is not digested. Neither are hemicellulose, pectin and lignin. And that is the biological definition of fiber in human nutrition: indigestible plant matter, particularly indigestible plant carbohydrates.
Fiber is a component of plant foods
