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78.4 - Summer 2005
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> Summer 2005 > News

History of Seed Size Yields Valuable Insight
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By Jonathan Schwank

The seeds of Carthamus lanatus, a member of the daisy family, were among thousands examined in this study.
The seeds of Carthamus lanatus, a member of the daisy family, were among thousands examined in this study. (Credit: Ron Oldfield)

Millions of years of evolution have produced plants whose seed masses span 11.5 orders of magnitude. The extremes range from the miniscule seeds of orchids weighing just a small fraction of a milligram in mass to the enormous seeds of the double coconut weighing as much as 20 kilograms. To better understand the history of seed size and its implications, a group of researchers including Campbell Webb, associate research scientist of ecology and evolutionary biology, analyzed seed mass data for 12,987 species of modern and prehistoric plants.

The extensive study identified the divergence of angiosperms (flowering plants) and gymnosperms (non-flowering plants) as the largest contributor to the modern diversity in seed mass. Reduced costs of pollen capture, decreased gametophyte size, and early abortion may have enabled angiosperms to produce seeds considerably smaller than those produced by gymnosperms. The difference in seed size between angiosperms and gymnosperms is already evident in the fossil record. Data of modern plants further demonstrates this significant disparity, in which the smallest gymnosperm seeds are almost 10,000 times more massive than the smallest angiosperm seeds.

Also of note, no specific time period appears to have made a particularly large impact on the seed size deviation. As Webb explains, “Divergences in seed size seem [to be] sprinkled evenly through time, rather than concentrated at a few particular geological events.”

The researchers did find that major shifts in seed size correlate with major shifts in plant stature. Webb “had expected there to be more independence between these traits,” since optimal seed size for a given species depends on a variety of ecological variables. Though this general correlation of seed size with plant size is surprising, it remains consistent with cross-species studies and can be explained by evolutionary theory.

Reference:

Moles, A.T., et al. (2005). A Brief History of Seed Size. Science, 307, (5709), 576-580.
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