Sunday, February 25, 2024

Fruits of the Rose Family—a Cornucopia!

How many Rosaceous fruits are in this painting? Answer at end of post. (Still Life with Fruit by Severin Roesen, 1852; Smithsonian Open Access)
The Rosaceae is a large cosmopolitan family of plants, with c. 3000 species scattered across every continent except Antarctica. It's best represented in the Northern Hemisphere, mainly in temperate habitats. This is a family much loved by humans. Gardeners have grown the "queen of the flowers" (the rose) for at least 5000 years, and we've enjoyed the diverse delicious fruits even longer.

Members of a plant family, even a large one, are supposed to be similar and often they are. For example, flowers of the vast majority of species in the Rose Family (excluding ornamentals) have five showy-but-simple petals surrounded by five green sepals. Inside the petals are numerous pollen-producing stamens surrounding one-to-many ovaries, each of which contains one or two ovules awaiting fertilization.
A typical Rosaceous flower: 5 sepals (green tips visible), 5 petals, many stamens (anthers at tips), and a cluster of numerous ovaries. (Wild Strawberry, Fragaria virginiana. Matt Lavin photo)
Unlike the flowers of the Rose Family—so similar and so simple—the fruits are diverse and often complex, in fact notoriously so. For centuries botanists have tried to subdivide the family based on fruit type. They've always failed.

By definition, fruits are mature ovaries containing mature ovules (seeds). But there can be so much more—thick flesh, plumose tails, tough skins, rock-hard coats, accessory structures, and aggregation. The Rose Family includes all of these! Though this diversity frustrates plant taxonomists, the rest of us can enjoy it :)

It would be foolish to try to cover the full range of Rosaceous fruit diversity. Readers would begin to fall away less than halfway through. Instead here are some favorites, starting with complicated and delicious types, and finishing with one that's quite simple and not at all tasty, but spectacular in its own way.

But first ... do you know which Rosaceous fruit type is most widely consumed by humans?
Our favorite fruit in the Rose Family is a pome, from Old French "pome" meaning apple (source).
Malus domesticus, the domestic apple tree, probably originated in the mountains of Central Asia. Now it's represented by thousands of cultivars grown in temperate regions worldwide. The apple itself is a pome, which botanists define as a fruit with "a central core containing multiple small seeds, which is enveloped by a tough membrane and surrounded by an edible layer of flesh." Technically speaking, the thick fleshy layer is an accessory structure.

Slicing an apple in half lengthwise shows how evolution elaborated on the basic seeds-in-mature-ovary structure in creating the pome.
Granny Smith apples. Photo by benjamint444, labels added.
The pome is unique to the Rose Family, but not just to apples. Pears and quinces also bear pomes, as do many native species. Their pomes are small, but look closely—they are indeed tiny apples.
Pomes of four species of hawthorne (Crataegus), c. 1 cm diameter; by Nadiatalent.
Another popular fruit type with a delicious accessory structure is the drupe, also known as stone fruit.
Domestic cherry—one of many delicious drupes in the Rose Family (source unknown). 
The genus Prunus includes what appears to be a diversity of fruits: cherries, plums, peaches, nectarines, apricots, and almonds! But in fact they all are drupes, defined as "fruit in which an outer fleshy part surrounds a single shell (the pit or stone) with a seed (kernel) inside" (source). [Unlike pomes, drupes are not limited to the Rose Family. For example, olives and dates are drupes.]
A sliced peach reveals tasty flesh surrounding the pit. DG Passmore, 1895, National Agricultural Library.
The almond tree, Prunus dulcis, also bears drupes. The almond itself is a seed (source; labels added).
Drupes are well represented in the wild, for example our many species of wild cherries.
Harvest time! Chokecherries, Prunus virginiana, by Matt Lavin.
"Thousands of drupes readied to make chokecherry whatever" says Matt. Would that be jelly? wine?
Now we advance to a higher level of complexity—a fruit that develops from multiple ovaries of a single flower. A tasty but controversial example is the strawberry (actually not a berry but that's not the controversy).
Wood Strawberry, Fragaria vesca (Jakob Sturm, 1798, BHL via Flickr). Question added.
Strawberry flowers are typical of the Rose Family (see above). But after fertilization, things get interesting. The many ovaries, each containing a single ovule, mature to become achenes—dry one-seeded fruits (g/G in illustration). At the same time the flower base (receptacle) grows, becoming a red mass of yummy flesh with the achenes embedded on its surface (f in illustration).

Herein lies the controversy: What is the fruit of the strawberry? Is it the fleshy globe adorned with achenes, or the achenes themselves? Some botanists rage over this, insisting achenes are the true fruits (in Wikipedia for example). Others think this silly, and simply refer to strawberries as aggregate fruits (my preference).
Young strawberry, with styles still present on maturing achenes (Olivier via Flickr).
Wild Strawberry, Fragaria virginiana (JW Frank via Flickr).
As promised, the final fruit is simple but beautiful—a single achene, specifically that of Mountain Mahogany (genus Cercocarpus). These shrubs and small trees of the arid American West are members of the Rose Family. But no matter how long one stares at their little flowers, it's hard to see roses.
Flowers of Birchleaf Mountain Mahogany (C. betulifolia) are just 5 mm across and have NO petals. The sepals form a cup with many stamens surrounding a single pistil (Joe Decruyenaere via Flickr).
A Mountain Mahogany flower may be humble, but not the resulting fruit. The pistil and its ovule become an achene with a long persistent feathery style, ready to fly with the wind. En masse, these "seed tails" transform the plant that bears them.
Fruits of Mountain Mahogany (C. montanus) preparing to launch; seed tails to 8 cm long (Matt Lavin).
Mountain Mahogany transformed by seed tails. Achenes can be spectacular! (C. ledifolius, Matt Lavin)
Now we return to where we started—"How many Rosaceous fruits are in Roesen's painting?" The answer is "many" (exact number depends on your opinion regarding aggregate fruits). I found pomes (apples), drupes (peaches, plums, cherries, maybe nectarines), aggregates of drupes (blackberries), and aggregates of achenes (strawberries). Did I miss anything?

This has been quite a long post, I agree. But I would be remiss if I were to omit Robert Frost's thoughts on the subject.

Sources (in addition to links in post)

Rosaceae in Flora of North America, which includes a list of fruit types:
"... achenes aggregated or not, follicles aggregated or not, drupes aggregated or not, aggregated nutlets, pomes, aggregated drupelets, or capsules; sometimes involving accessory organs, for example, hypanthium, torus."

Haywood, VH. 1978. Flowering Plants of the World. Oxford University Press.

Judd, WS, et al. 2002. Plant Systematics, 2nd ed. Sinauer Associates, Inc.

Tuesday, February 13, 2024

South Dakota Tree-Following—Plains Cottonwood

"A circle of cottonwood-leaf toy tipis made by Indian children of Plains tribes ... These they made in numbers and placed them in circles like the camp circle of their tribe." (Gilmore 1919)
Last month I launched a project to get to know the trees of South Dakota, starting with Black Hills Spruce, the state tree. This month I chose Plains Cottonwood, which was a strong state-tree competitor and rightfully so. Plains Indians relied on it for construction materials, fuel for heat, and winter food for horses. Fur traders built stockades and boats from cottonwood trunks. And as early travelers slowly made their way across the prairies, the occasional tree offered a welcome bit of shade. That hasn't changed.
Plains Cottonwood, western South Dakota. I ate lunch in its shade every day during a grassland project.
While looking for information, I came across a thought-provoking article: "Cottonwood Houses, Cottonwood Stars" (2014). Much of it is included here. Sometimes a look back shows what has happened since. Sometimes we can undo a bit of that.

In the early 1900s, ethnobotanist Melvin Gilmore visited with elderly Indians of the Great Plains, specifically those who had gathered native plants and still knew the old names and uses. He hoped to record this knowledge "while it may still be obtained, before the death of all the old people who alone possess it.” As it turned out, those old people were eager to share so that “future generations of their own people as well as the white people may know and understand their manner of life."

Gilbert observed and described construction of toy tipis from the broad deltoid leaves of Plains Cottonwood, Populus deltoides ssp. occidentalis. Ten years ago, I carefully followed his instructions:

"They split a leaf a short distance down from the tip along the midrib; at equal distances from the tip they tore across from the margin slightly; then, bending back the margin above the rents for the smoke flaps, and drawing together the leaf-margins below the rents and fastening them with a splinter or a thorn, they had a toy tipi."
Smoke flaps regulate draft and ventilate the tipi, especially smoke from the fire.
My camp circle.

Another gift of the Cottonwood are the stars concealed in its twigs. Kathleen Cain learned this as a child in rural Nebraska, from her father.

"You have to find [a twig] with a sturdy knuckle ... You have to cut cleanly ... One cut is best ... He turned the twig so I could gaze directly into its center. Running crosswise through the middle of the small piece of wood, the cut revealed a reddish-brown and nearly perfect five-pointed star." (Cain 2007)
A sturdy knuckle—the joint between two years’ growth. If you want to look for a cottonwood star, other species will work also. This is P. acuminata.

This cottonwood star is 6 mm across.

As I learned from my cottonwood projects, being a child again is therapeutic. As if to drive the point home, this showed up in The New York Times today:

"If I had influence with the good fairy who is supposed to preside over the christening of all children, I should ask that her gift to each child in the world be a sense of wonder so indestructible that it would last throughout life, as an unfailing antidote against the boredom and disenchantments of later years, the sterile preoccupation with things that are artificial, the alienation from the sources of our strength." Rachel Carson, The Sense of Wonder


Cain, K. 2007. The cottonwood tree; an American champion. Boulder, CO: Johnson Books.

Cottonwood Houses, Cottonwood Stars. November 2014.

Gilmore, MR. 1919. Use of plants by the Indians of the Missouri River region. Bureau of American Ethnology.

Johnson, W. Carter, and Knight, Dennis H. 2022. Ecology of Dakota Landscapes; past, present, and future. Yale University Press (in print and ebook format).

This is my report for the February gathering of tree followers, kindly hosted by The Squirrelbasket. If you'd like to join us, you can learn more here.