Seeds, corms, bulbs, tubers and rhizomes are all means of plant reproduction. Growers need to know the differences amongst them, as they all require different treatment.
What is a seed?
A seed is the means by which flowering plants (angiosperms) and plants with cones (gymnosperms) make sure that their species continues. A seed is an organ of reproduction. It contains a tiny plant or embryo and, sometimes, the endosperm or stored food for the plant and they are covered in a seed coat. The embryo consists of a root, a stem and one or more leaves.
The variety in size and shape of seeds is almost endless. Seeds also vary greatly in the way that they travel from the mother plant. Some are carried by the wind or birds, some attach themselves to animals or insects, some float on water and some plants burst open to scatter their seeds.
A corm is a modified underground stem that has been developed to store food. It is covered by a tunic. At the bottom of the corm there is a basal plate and the roots emerge from that. A corm has one or more growing points at the top.
Popular South African flowers, gladioli and freesias, are amongst the flowers that grow out of corms. Yams and Chinese water chestnuts are amongst some of the corms that people eat. The African Potato (Hypoxis hemerocallidea) grows out of a corm and is valued for its medicinal properties.
A bulb is also a modified stem, which is the resting stage of certain seed plants. It is shaped like a globe, which narrows to a point at the top. The stem and leaves grow out of this point.
There is a flat part at the bottom, called the basal plate, from which the roots grow.
Sometimes the bulb has a covering of a thin, papery layer ~ like that of the onion. Other bulbs, such as the lilies, do not have this tunic.
Bulbs are made up of rings, which are called scales and which are actually modified leaves, in which they store food.
Well known indigenous flowers from bulbs include Sparaxis, ixia and babiana. Some flower bulbs, such as the chincherinchee, are poisonous to humans and animals.
Edible bulbs include onion, garlic, shallots and leeks.
A rhizome, which is sometimes called a creeping rootstalk, is a stem which grows horizontally underground and which is able to produce the shoot and roots of a new plant. The plant uses the rhizome to store starch and protein.
Bamboos, water lilies and many ferns grow from rhizomes. Rhizomes that we eat include ginger, turmeric and lotus.
A tuber is yet another specialized stem used as storage by a plant while it is resting. Tubers grow under the soil and are usually quite short. They have several growing points, called eyes. They do not have tunics or basal plates.
Potatoes are the best known tubers. The Jerusalem artichoke is also a tuber.
Tuberous roots, such as sweet potatoes and cassavas, are different from tubers.
There are members of our communities who know of seeds, corms, rhizomes and tubers that can be found in the veld that are safe to eat.
A seedling is a very young plant that has grown from an embryo in a seed. When the temperature, water and oxygen are right for that particular species, a seed will sprout. We call this germination.
Smallholders will buy vegetable seedlings to save time. Vegetables planted from seed will take weeks longer to reach harvest state that those planted as seedlings. The nursery has taken the risk out of planting, as you are buying the successful plants. You cannot control the weather and the nursery staff has taken care of disease and pests.
On the other hand, some seeds germinate relatively quickly, such beans. Planting from seed also makes succession planting easier.
If a small farmer is planting vegetables to sell or to help feed the poor, it will prove too expensive to grow from seedling.
This is part of a five-part series on seeds and plant nutrition.
Part Two: Different types of seeds
Part Three: Collecting, saving, storing and swapping seeds
Part Five: Plant nutrition: Using foliar feeding