Plants Be Complicated: the Rosaceae, part one

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serviceberry (Amelanchier species)

When writing about plant families, I like to quote a few statistics, but that isn’t so easy with the Rosaceae. A web search of “number of species in Rosaceae” yields the following:

 

 

 

  • “…4,828 known species in 91 genera” (Wikipedia)
  • “…9,404 scientific plant names of species rank for the family Rosaceae. Of these 1,966 are accepted species names….[there are] a further 2,836 scientific plant names of infraspecific rank…” (The Plant List)
  • “…some 2,500 species in more than 90 genera.” (Britannica.com)
  • “Worldwide, there are about 100 genera and 3,000 species.” (Wildflowers and Weeds)
  • “The Rosaceae comprises approximately 3,400 species…” (USU Herbarium)
  • “…comprising about 100 genera and 3,000 species.” (University of Hawaii)

You get the idea. But why is it so hard to pin down? For one thing, people are still discovering previously undocumented plants. More significantly, studies of genetics and evolutionary history change our understanding of how organisms are related to one another, leading to changes in how they’re classified. Sometimes several species are lumped into one; other times, one species is split into several, leading to the creation of new species.*

When it comes to the Rosaceae, there’s another factor: apomixis. Simply put, this is the ability of a flowering plant to reproduce via seed that has formed asexually. Which sounds wrong, because we all know that seeds are formed when ovules are fertilized by pollen…right?

But then, as a botanist friend put it, “plants be complicated.”

There are several mechanisms by which apomixis occur, and a summary is beyond the scope of this blog (that is, I don’t fully understand it yet myself), but the upshot here is that apomixis kinda-sorta might result in the formation of new species; at the least, it muddies our ability to trace genetic relationships:

Apomixis also frequently leads to the formation and maintenance of numerous morphologically distinct, yet interfertile, varieties growing true to type from seed. The taxonomy of such agamic complexes can be a difficult and contentious task…   –Understanding Apomixis: Recent Advances and Remaining Conundrums
Ross A. Bicknella and Anna M. Koltunowb
(full article)

According to the Wikipedia entry, the genus Cotoneaster contains between 70 and 300 species, Crataegus between 200 and 1,000, and Rubus may have thousands.

It seems futile, then, to say how many species are in the Rosaceae, but I’ll continue with a few more statistics. According to BONAP, the Rosaceae is the fifth largest family in North America (counting natives only), with 664 species. The Maryland Biodiversity Project has records for 142 species, about half of which can be found in the piedmont. About 40 of the Maryland species are aliens, eight are listed S1, three are S2, and five are S3.

Species in the Roscaeae occur worldwide except the arctic and antarctic, with the greatest diversity found in the northern hemisphere. The flowers almost always consist of five sepals, 5 petals, and many stamens (ten or more), all fused together at their base into a cup-shaped structure termed a hypanthium. The flowers are borne in racemes, spikes, or heads, or sometimes singly. Leaves are usually arranged alternately on the stem, can be simple or pinnately compound, usually have stipules, and often have toothed margins. Herbaceous plants are usually perennials, and woody plants are usually deciduous.

This is a family of major economic importance, and not just for cut roses in the flower industry. More on that next time.


*interesting article: Whence Lumpers and Splitters? (National Center for Science Education)

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