Most plants reproduce sexually, which leads to the formation of seeds. By the nature of this process the seeds produced show a variation
in characteristics to a greater or lesser extent. Typically plants
produced from seed will not be uniform in their growth and will exhibit
differences in size, flower colour, etc. This variation can be controlled
by skilled plant breeders to the extent that a high degree
of uniformity can be achieved in bedding, e.g. for flower colour, and
vegetable seeds, e.g. for size and ‘once over harvesting’ (see hybrids).
is the production of new individuals by the fusion
of a nucleus from the male (in pollen) and that of a female (in the ovule)
to form a zygote (see Plant
and Pollination and
Seeds germinate when provided with the right conditions regarding:
- air (oxygen)
and, for some, an exposure to light, or, for others, an absence of light, as
described in detail in Plant development
. In some cases germination
will not take place even if otherwise favourable conditions prevail
(consider the seeds that fall into the warm, moist soil in the autumn
but do not germinate until the following spring or later). These seeds
are exhibiting dormancy, which has to be broken to allow germination
to occur. This is a survival mechanism that helps prevent
the seed start germinating just when conditions are about to become
unfavourable for growth.
is a mechanism, such as a hard seed coat, which
has to be broken before water and oxygen can get in. Rather than wait,
growers can speed up the process by scarification,
e.g. sand papering
or filing the coat; ‘chipping’ or ‘nicking’ it with a knife or, as in the
trade, by adding acids. Water can then get in quickly through the thin
or damaged seed coat and start the germination process. For many
seeds simply adding hot water is sufficient to remove the waterproofing
qualities of the seed and let water in.
includes the effect of abscisic acid in the
dry seed which inhibits development of the embryo. Germination cannot begin until its concentration is reduced. In temperate areas an
exposure to prolonged cold gradually destroys the inhibitor. Growers
can overcome this mechanism by exposing the seed to cold artificially. Stratification
is the usual method of overcoming this form of dormancy.
The seeds are placed in layers of moist sphagnum moss and grit within
a polythene bag. The seeds are allowed to take up water in the warm,
but once swollen the bag and its contents are chilled but not frozen. For
some species, they are ready to germinate after a month, others take
much longer. Once the dormancy of most species is broken they do not
develop further until all the normal requirements for germination are
met. Care needs to be taken because some species start to germinate
once their chilling period has been experienced.
Many seeds develop dormancy on storage. It is possible to avoid the
problem by sowing ‘green’ seed. Seed can be collected when it is mature
and with adequate food reserves, but before the dormancy mechanisms
become established (soft seed coat, low abscisic acid), and sown straight
especially vegetable and flower seeds, has the
advantage of convenience and the protection of the regulations. A check of the date should always be made to ensure that the
seeds are from the last seed harvest. The seeds are usually supplied
in foil packets. Once opened the seeds deteriorate rapidly so should
be sown immediately but, so long as they are kept dry and cold in a
resealed packet, most seeds will remain viable for a year and some,
often the larger seeds, for many years.
There are difficulties when it comes to seeds from trees or shrubs
because there are fewer regulations to protect the buyer. In the
preparation of seeds for sale, the drying process used often:
- increases the dormancy effect (harder coats);
- adversely affects the energy reserves;
- damages the embryo;
so reducing seed viability.
Seeds, especially finer ones, are often coated (with a clay) to make
sowing easier and more precise. This pelleted seed
can help reduce
wastage. Likewise, water-soluble seed tapes can be used. A gel (or
wallpaper paste) containing seeds can be used for fluid drilling;
is squeezed out of a plastic bag like icing a cake. Some seeds that are
difficult to germinate can be primed;
the germination process is started
but then arrested. The dried seed purchased can be drilled or sown as
normal and rapid and reliable germination follows.
can prove to be cheaper. Although there are attractions
in keeping their own seed,
growers need to be aware of difficulties
associated with seed variation and the risk of disease. Seed collectors can ensure they take the seed at the ideal time,
especially when the intention is to avoid dormancy problems, and care
can be taken in drying so as to minimize loss of viability. Where the
hardiness of the plant is in doubt then, although a hardy parent does not always produce hardy seed, the chances of success are raised by
taking seed from a known hardy specimen. There are other advantages,
particularly when it comes to trees and shrubs, because seed can
be taken from desirable forms, beneficial even though there will be
variation in the offspring.
The majority of seed should be collected as they ripen. Seed in dry fruits
should be collected on a dry day. It should be noted that when enclosed
in a fruit, the seed is ready to collect before the fruit matures ready for
dispersal. A collecting bag, plastic rather than cloth, to keep hands free
is an advantage and it is essential to label samples with the name of
the plant and from where collected. The seeds should be kept in small
batches and kept cool (to prevent the embryo from heating up). The seed
should be prepared for stratification and/or sown as quickly as possible
for maximum benefit.
Dry seed needs to be prepared from the material collected. Flower stems
can be tied lightly then hung upside down in a dry place with a brown
paper bag over them; shake from time to time to collect the seed. Large
seed heads should be broken up into trays on paper and left to dry.
Cones or small seed heads collected when nearly dry should be placed
in an open paper bag and left to complete their drying gently. The flesh
of fruits, which often contains germination inhibitor, should have the
majority of the flesh removed before being squeezed through a sieve
with a presser board. The seeds with any remaining flesh should then
be put in a jar of warm water and soaked for a few days after which the
water is poured off. This is repeated until the flesh has been removed.
The remaining skin is then picked off and the seeds dried. Sieves can be
used to remove any superfluous pieces before putting the seed into paper
packets ready for sowing.
Storing seed which is to be used within a few days requires little more
than keeping them at room temperature in a polythene bag to maintain
the moisture levels at which they were collected. If they are to be kept
for a few weeks then the seeds need to be stored cool, but not frozen.
Seed to be stored for longer periods than this, as when commercially
produced for sale, is dried, placed in air proof packets (commonly foil),
vacuumed to remove air and kept cool. Some of the large fleshy seeds,
such as lilies and hellebores, are best left to mature and collected before
they are dispersed. Other seed such as anemones is collected and sown
‘green’, i.e. before maturing.