Answer: Why do beans and peas grow this way?

To tell the truth... 

this Challenge started with an interesting observation.  This is one of my bean plants, growing quite well up the bamboo pole last week.  

Eventually, it will make it to the top (12 feet, 4 meters), and maybe a bit beyond. 

Here's that same plant 1 week later with significant additional twining.  

As you can see, the "wrapping growth" has continued.  

Watching the beans growing like this leads me to ask a few Research Questions about how beans and peas do this.  

1. How do the plants find what to climb on?   I mean, they  don't have eyes, so... how do they find the closest support to climb up?  

I started this search with a very simple query: 

     [ how does a bean plant wrap around a support ] 

which gave me a "featured snippet" that looks like this: 

Featured snippets are in a special box at the top of your search results with a text description above the link. Most featured snippets only contain one listing, a fragment of the full page that best matches the question you've asked.  

If you click through to the landing page ("Why Vines Twine"), the snippet will be selected and highlighted for you, like this: 

In this case, the snippet is from Melinda Myers gardening site.  She's a well-known gardener, and the rest of this page gives us some helpful ideas about what's going on.  

Melinda points out that when a stem touches something, the cells on the opposite side of the stem from the touch grow longer.  This causes the curling effect we see. 

This page also mentions a few other ideas:  circumnutation, thigmotropism and tendril.  A quick..

     [ define circumnutation ] 

tells me that this is "...a movement of the growing portions of a plant to form spirals, irregular curves, or ellipses — compare nutation."   I'm never one to pass up a definition, so I look up "nutation" which is "the circular swaying movement of the tip of a growing shoot."

Swaying?  This word suggests that the plant is moving actively.  This makes me think that perhaps there's a video that would be worth watching.  

Over to YouTube with the search for: 

     [ circumnutation bean] 

and you'll find lots of time-lapse videos.  Here's one that shows several bean stems waving around in a circular motion until one tendril finally finds and latches onto a neighbor.  

As you can see, the bean stalk grows upward, waving in a circular pattern until it hits something.  One there's a contact, the wrapping behavior begins.  

Continuing in this vein:  

     [ define thigmotropism ] 

We learn that thigmotropism is a directional growth movement which occurs as a growth response to a touch stimulus. Thigmotropism is typically found in twining plants and tendrils, however plant biologists have also found thigmotropic responses in flowering plants and fungi

We've also learned a new word:  tendril.  What's a tendril?  Read on... 

2. Speaking of which... What part of the plant does the searching for the nearest support? Is there a specific name for this?  

A tendril, that word we just picked-up in our reading is a "slender threadlike appendage of a climbing plant, often growing in a spiral form, that stretches out and twines around any suitable support." 

A search on Google Scholar for [ tendril growth ] leads to a bunch of resources, many of which have a huge number of marvelous insights.  Some of these papers are a little difficult to read, but here's one I have to quote to you... 

 "Successful speciation in climbers is correlated with the development of specialized climbing strategies such as tendrils, i.e., filiform organs with the ability to twine around other structures through helical growth. Tendrils are derived from a variety of morphological structures, e.g., stems, leaves, and inflorescences, and are found in various plant families. In fact, tendrils are distributed throughout the angiosperm phylogeny, from magnoliids to asterids, making these structures a great model to study convergent evolution..." 

Sousa-Baena, M. S., Sinha, N. R., Hernandes-Lopes, J., & Lohmann, L. G. (2018). Convergent evolution and the diverse ontogenetic origins of tendrils in angiosperms. Frontiers in plant science, 9, 403.

In other words, tendrils are the parts of the plant that do the searching for something to hang onto.  What's more, they have evolved multiple times in many different kinds of plants.  Tendrils have come from all kinds of plant parts (stems, leaves, flowers, etc.), which suggests that they'd be great to study how different species of plants come to the same kind of solution for finding support (convergent evolution).  

As I read through these papers, I was mostly just skimming...  But I was able to find all the information I needed.  

3. How / when does the plant decide to start hunting for support?  My beans didn't seem to start immediately looking for a support, so how do they know when to search?  

This Challenge is really about understanding how a bean plant sends out a tendril.  As we learned in the above quote, tendrils can arise from many different parts of the plant (stems, leaves, flowers, etc.)... but what about just beans?  

I didn't really know how to start this search.  What kinds of query terms should one use here?  

So, I started with this fairly straightforward query: 

     [ origin of tendril growth ] 

The first result is a fairly technical article "Convergent Evolution and the Diverse Ontogenetic Origins of Tendrils in Angiosperms" which gives us a new word that might be useful: ontogenetic (which means "the origination and development of an organism from the time of fertilization of the egg to adult" -- that is, the ways in which an organism grows and changes over its lifetime).  

In particular we want to know the "ontogenetic development of tendrils" to learn how/when/where a tendril starts to grow.  

This long (and technical) article is full of great insights, although you have to be willing to read through the thicket of prose.  In particular, we're interested in how beans grow tendrils.  In order to search inside the paper, you have to know that my runner bean is called Phaseolus coccineus, a member of the legume family, Fabaceae.  When I do my Control-F for Fabaceae and look for tendril near that word, I find this remarkable sentence (slightly simplified to focus on the beans' strategies): 

The orders with larger numbers of tendrilling strategies are Fabales and Asterales  In the ... Fabaceae ... evolved tendrils, comprising three different strategies: (i) whole leaves modified into tendrils (found in the Fabaceae exclusively); (ii) terminal leaflets modified into tendrils; and (iii) shoots modified into tendrils... 

This is illustrated in a diagram in the paper (modified here for clarity):  

This is a tendril springing from the base of a leaf, a kind of generic bean tendril growth pattern.  And, as seen in the Phaseolus vulgaris in my garden: 

Here the arrow points to a tendril that is wrapping around the support string (at the very top of the photo), while the stalk on the right holds a large leaf (just out of the photo).  

Obviously, this tendril doesn't happen until at least the second node on the stem has formed. THEN it starts growing.  And, of course, each climbing plant is a bit different, some starts twining at the very beginning of their brief lives, some emit tendrils late, and some emit tendrils only when the occasion calls for it.  For instance, Poison Oak (Toxicodendron diversilobum in California) can grow as either a shrub or a vine depending on the amount of sunshine it gets and the availability of support structures to climb.  A small poison oak plant will become a vine, with plenty of tendrils, if it's raised in a darker place (say, in the shade of larger plants) vs. a brighter location.  Of course, if it's in the shade of other plants, it will grow up along those plants to make the poison oak vine so be-hated by hikers on Californian trails. 

 

4. What do you call this behavior?  (Note: there’s a specific term for a lot of these concepts, in particular, the movement of a plant wrapping around something for support.) 

My query: 

     [ plant tendrils searching for support ] 

quickly leads me to yet another term I hadn't seen before, circumnutation, a term that  I learned describes the apex of a stem or other growing part of a plant as it bends or moves around in an irregular circular or elliptical path.

Of course I had to do a search for: 

     [ circumnutation ] 

which leads to some more wonderful videos of plants waving their stalks around--circumnuating--in a circular pattern, searching for a support.  Here's another great one--you can see the stalk circling around until it hits the vertical support, when it begins its twining motion.  

It's worth nothing that not all supports are equally useful.  A support that's too large (say, a tree trunk) might not be able to support a twining plant like a bean or a pea.  To successfully attach a vine to a giant support (or a wall) requires another mechanism... adhesive roots or tendrils.  (See the paper Moving with Climbing Plants from Charles Darwin's time into the 21st Century for more details.) 

5. And... who coined that specific term?  (Can you find the coiner and where it was coined?)  

Easy: 

     [ circumnutation coined ] 

But Surprise, Surprise!  This quickly tells us that Charles Darwin coined this term in his book, The Power of Movement in Plants (Darwin, 1880), a publication which really kicked off the entire sub-area of studying how plants move.  

Search Lessons 

A few very clear lessons here:  

1.  Asking simple questions often leads to valuable results!  We were able to start off with a  [ how does a bean plant wrap around a support ]  and quickly extract information that told us a LOT about how plants twine their way up a support.  

2. Look specifically for terms you don't know, and then look them up.  I've said it before, but learning the definitions of unusual terms is a great way to get into a domain.  It's a superpower that you can have!  

3. Simple queries still work really well, especially when you have a specific question in mind.  

Hope you enjoyed this SearchResearch Challenge... I certainly learned about how and why plants twine and climb.  

Quick note:  Sorry this took so long to get to you.  I didn't PLAN for this to take an extra week.  It's just that these days of COVID and public strife end up pushing SRS to a slower work schedule.  I'll try to keep up!  Hope you're doing well during these trying days.  

Search on!