Most plants reproduce sexually, which leads to the formation of seeds (see p103). 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 (see p144) 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 p144).
Sexual reproduction 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 Chapters 7 and 10).
Physical dormancy 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.
Physiological dormancy 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 away.
Purchasing seeds, especially vegetable and flower seeds, has the advantage of convenience and the protection of the regulations (see p152). 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 (see p168).
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:
so reducing seed viability (see p152).
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; the gel 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.
Collecting seed 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 (see p144) and the risk of disease (see p147). 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.
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