Carpels vs. Carpals

A Human hand riddled with buckshot, as shown from one of the earliest human X-rays, taken by Michael Pupin in 1896

Some words are unrelated, yet curiously sympathetic – road and rode, wrote and rote, die and dye… Carpel and Carpal fall in that category. They sound the same, and both words have to do with individual parts that may function together. But any relationship has only to do with their etymological roots. Carpel comes to us through the association with fruit (karpos Gr = fruit) while carpal evolved as a medical term that refers to the wrist (also karpos in Gr = wrist), which tells us the confusion and connection go back thousands of years. In contemporary English, spelling is defining. The blocky wrist bones so clearly shown in the early X-ray (1896) on display in The Huntington’s exhibit “Beautiful Science” are carpals.

Beyond the Greek “karp” that anchors hands and fingers, botanists have adopted the fruit-related Greek “karp” in the form of carpel, as the term for each segment in a pistil. In ‘Valencia’ oranges from the Huntington citrus groves, you see segments (i.e. carpels) in cross-section.

‘Valencia’ oranges, 2018 crop and the more recent (green) 2019 fruit

But looking at Oranges gets us ahead of the story. To botanists, a pistil (therefore, a fruit) is made of one or more leaves. Many flowers, such as legumes, have a simple pistil made of a single carpel – which botanists consider a fertile leaf. Opening a Sugar Snap Pea gives clear evidence as to that relationship, in which the single carpel almost looks like a leaf

Peas have one pistil made of a single carpel. Other kinds of plants, like Magnolia and Peony, have more than one pistil in a single flower, each pistil remaining a separate structure, a separate carpel.

The light green structures spiraling toward the apex of the Magnolia flower are its numerous, individual pistils – each a single carpel
Magnolia, mature fruiting structure – showing seed (with red seed coats), each emerging from its now-dry pistil. The small scars on the rich brown supporting stem (the peduncle) show where stamens had been attached in the flower.

So carpels can be confusing. Thanksfully, peas are straightforward: one flower makes one pistil which consists of one carpel. And the Magnolia is reasonably intelligible: one flower makes many pistils, each pistil made of one carpel. In citrus, we have to get used to the very normal fact that one flower has one pistil, which can be made of many segments, i.e. many carpels. Botanists get a lot of mileage out of knowing how many segments are in each fruit, because internal fruit structure tends to be something that remains standard for plant groups. Since it’s usually easy to determine the number of segments, that characteristic can help significantly when we are trying to identify a plant.

This brings another useful pair of botanical words into play in the discussion – connate and adnate. When several carpels grow beautifully together in a pistil, making a single fruit that is segmented internally, we say the carpels are connate – which means that similar organs have melded, i.e. grown together as one. Connate shows up in morphological discussions, as does its alter ego, adnate. Adnate means that two different kinds of organs have grown together seamlessly, like a leaf growing attached solidly to a stem, or a stamen attached to a petal.

In lemons and oranges, the many carpels normally form a single fruit that is seamless from the outside. But things can happen. Growth of a developing pistil can be impacted, such that the carpels fail to become fully connate, which means you may find lemons that show incomplete union.

When there is a disturbance in the growth pattern of Lemons (invasion of the developing pistil by an insect, for example), you might find fruit that break into individual carpels at some level.

In Buddha’s Hand lemon, the failure is genetically-based. Carpels never grow fully connate.

A genetic difference means that Carpels in Buddha’s Hand Lemons naturally fail to grow together, thus showing segments individually.

This isn’t the end of terminology and confusion in the world of Carpology (yes, it is a word). There’s a full range of possibilities. And there are more etymological quandaries. At some future point, we’ll look into the relationships between pistil and pistol. In the meantime, check out some other examples below.

Weirder things can happen than the breaking apart of Citrus segments. In this Mandarin, another compound pistil formed inside the main pistil, a fruit within a fruit.
Australian finger lime is a member of the Citrus family that produces unicarpellate pistils (the pistil has only one segment, i.e. leaf)
Passion fruit (Passiflora) are made of three carpels that form a hollow vesicle, bearing seed along three fertile zones (each fertile strip is a placenta) along the inner fruit wall.
Cucurbits make fruit resembling that of Passifloras – seed usually form along three placentae that form along the inner periphery.

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