June 18th – back in the “turf” again, this time with midshipman Philip Gidley King on horseback.:
“King came & spent the day with me; we both on horseback started for the old forest.— He shot some birds & as is generally the case I found many interesting animals of the lower classes.” (June 18)
Shooting and riding were a couple of Darwin’s favorite things back home in England – probably pretty typical for an English gentleman such as himself. The fact that he was still distracted by insects while King shot at birds today, is interesting. It is part of his transition from “an idle sporting man” (as he called himself) to a naturalist. His tastes were changing.
Darwin’s attention then turned to the trees – specifically the palm trees:
“We found a little Palm tree, only a few inches in circumference, which I believe to be 305 years old.— I judge of this from its number of rings, each of which I imagine marks a year.” (June 18)
Palm trees on Ipanema Beach in Brazil (from LonelyPlanet.com):
Alas, this is one of those situations where a little knowledge led Darwin astray. Dendrochronology, the study of tree rings, is a fascinating and fruitful field (believe me – I spent several years in graduate school measuring and analyzing tree rings). Rings are produced by some trees every year (with the width of the ring depending on several factors including climate conditions). This “secondary growth” is responsible for increasing the overall diameter of the tree trunk and branches. “Primary growth”, by the way, is the vertical growth of the plant.
Now here is the catch – all vascular plants exhibit primary growth, however, secondary growth is only found in some plants. The gymnosperms, such as pines, cedars, ginkgos, and cycads, all exhibit secondary growth and produce rings. In fact, the “soft woods” such as hemlock, fir and pine, produce some of the best rings (as seen below).
Rings in a Red Pine (from the Ultimate Tree Ring Page – the ultimate tree ring resource)
The angiosperms (flowering plants) can be divided into two groups – the dicots and monocots (defined by the number of embryonic leaves (cotyledons) on the seed. Dicots have two leaves and monocots one. Most of the things that we think of as flowering trees, such as magnolia, dogwood, and birch trees, belong to the dicots. These trees also produce rings (though they are not always as prominent as the rings in the gymnosperms).
The monocots include plants such as lilies, orchids, and tulips, as well as the grasses (order Poales) and the palms (order Arecales). And – you guessed it – monocots don’t have secondary growth and therefore do not produce tree rings. So, alas, Darwin’s 305 “rings” in his palm tree were an illusion. Hey – you can’t be perfect all the time!
Cross section of a “ringless” coconut palm tree (from the FAO)
You can’t really knock Darwin – he was applying his knowledge of home to the exotic forests of Brazil. In Britain, most trees have rings, so there is no reason to expect that palms wouldn’t have them, too.
On the way home from the forest, Darwin and King run into Darwin’s recent hunting buddy – the nameless padre (described earlier this month as a “very rich man & a great favourite of the last queens” (June 4)) and his pack of hunting dogs (which Darwin describes affectionately in Darwin on a Hunting Expedition. He writes:
“On the road home I overtook my old friend the Padre, returning with his dogs from the Gavia.— He presented to me a magnificent specimen of the little once, which after five hours hunting, he had succeeded in shooting.” (June 18)
I’m not quite sure what the “little once” is? Maybe a typo (“little one”) referring to a bird like the ones King was shooting at earlier? (RJV)