Why is seed size important
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Seeds can be dispersed in a number of different ways. They may be carried by wind, water or animals. Some plants even shoot the seeds out explosively. Seed size is an important factor.
The seeds of flowering plants vary in size. Some are as small as grains of salt (e.g Foxglove), while others may be almost the size of golfballs (e.g. Horse Chestnut conkers). The difference in size reflects differences in the amount of food reserves stored in the seed for the benefit of the embryo plant inside. Usually, the larger the seed, the more food reserves it contains. This allows the germinating seed and young seedling more time to grow. It can then become well established before it must begin manufacturing its own food. The longer a seedling has before it must become self sufficient, the greater chance it has of becoming successfully established.
However, there is a down-side to having large seeds. The larger and heavier the seed, the more difficult it becomes to disperse it effectively by wind, or explosive techniques. All of these require light seeds. Seeds such as Foxglove are minute and are easily blown about by the wind. Larger wind-dispersed seeds are generally heavier and therefore require features such as parachutes or wings to help keep them aloft. For example, Dandelion seeds have developed very light and fluffy parachute-like structures. These help the seeds to float in the wind and delays their fall to the ground. This delay allows the seeds to be carried further. The largest and heaviest wind-dispersed seeds, such as Sycamore cannot rely on hair-like parachutes to keep them airborne. They would have to be enormous to be effective. Instead they have developed a wing which causes them to spin through the air like mini helicopters. This again delays their fall.
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Seed Size Often Matters
Q. Why are some seeds huge, while others, which may produce similarly sized plants, are tiny?
A. Many reasons for a disparity between seed size and plant size have been suggested, and many aspects of plant development are thought to be related to seed size.
Larger seeds are better able to support themselves initially, while smaller seeds have a better chance for dispersal over a wide area, helping at least some seedlings survive.
Plants also have evolved different adaptations to their sprouting environments, helping some smaller seeds thrive in drier, less nutritious soils, while larger seeds often are able to take advantage of richer soils that hold water better.
Seed size is often related to relative growth speed; smaller seeds develop more quickly than larger ones. But not always: a 2012 study in the journal Ecology concluded that small-seeded species only sometimes possess additional adaptations for rapid growth over and above their general size advantage.
A study last year in the journal PLoS Biology found that a faster speed of evolution of new species in a broad range of plant types is correlated with smaller seed size. Small-seeded plants also tended to have shorter life spans, with the rapid turnover linked to more rapid evolution.
Small seeds can develop more quickly and spread more widely than large-seeded plant species, although bigger seeds can thrive in richer, wetter soils.