Lower surface (left) and upper surface (right) of alt-leaf dogwood
I’ve made a discovery, or at least I’ve discovered something that’s at least new to me, something that is totally mind blowing. Er, at least mind blowing to a certain type of mind that appreciates this sort of stuff. LENTICELS make an impervious structure – the bark – pervious to gases like CO2, O2, and water vapor. I first noticed there seemed to be a pattern in the density of lenticels on twigs when looking at alternate leaf dogwood at Rock Point in Burlington back in 2014 (see above). That blew my mind plenty.
See, plant cells aren’t just one-trick photosynthetic ponies. They require oxygen to utilize photosynthates in growth, repair, and other metabolic processes.Â Lenticels, which are abundant in roots but are also present in bark, permit oxygen flow into the tree to fuel these processes. At some point in the past I had found some cool research indicating that lenticel density can be correlated to soil moisture levels – the wetter the more lenticels. This makes sense because lenticels are breaks in a structure otherwise designed to prevent water loss. I knew there was variation between species in lenticel density, but then I discovered that habitat influenced density and there was even variation between the upper and lower surfaces.
My recent discovery pretty much knocked my socks right off. I shouldn’t have been that surprised given what I’d read about water’s influence on lenticel density, but it still was cool to see a pattern revealing an underlying order to the anatomy of plants. I was looking at some of my photos of Norway maple buds and noticed that there seems to be a significantly higher concentration of lenticels just below the terminal buds. The change in density indicates a change in ecological pressures. Here, the requirement for increased gas exchange must mean some requirement to fuel:
- photosynthesis (by allowing CO2 to enter) or
- metabolism (by allowing O2 to enter).
If photosynthesis was the culprit, then there should be variability along a gradient of more light to less light; the top of the twig should be more photosynthetically active than the bottom (see again, the photo of dogwood above showing the increased density of lenticels on the top). But photosynthesis rates in twigs should be pretty consistent the length of the current year’s growth and here the lenticels were concentrated at the tip of the shoot. The buds contain next year’s growth, tiny embryonic leaves, shoots, and flowers. And all those cells need to burn lots and lots of energy to support all that growth. By concentrating lenticels right near the buds, the new growth would have all the oxygen it needed to fuel cell division and growth!! So there we have it, increased lenticel density as correlated to photosynthesis in the dogwood and metabolism in the Norway maple.
Norway maple bud. Detail showing brownish orange fuzzy lenticels beneath the terminal bud on a Norway maple (Centennial Woods)