Friday, April 3, 2009

US Botanic Garden (1): carnivorous plants

I have always enjoyed plants. It is funny: even with an extensive background in biology, plants have not been a part of my training. In US graduate schools, there is a sharp divide between molecular and organismic biology; as I was on the molecular biology side of the divide, the action of higher plants and animals was basically deemed irrelevant. In molecular biology, especially as I didn't study the model plant organism Arabidopsis, there was no mention of plants since a very basic biology class in college. I have been trying to remedy that recently. I am lucky that I live pretty near the US Botanic Garden here in DC, so that I can go and visit to find inspiration and more subject matter for drawing.
Image: Nepenthes x ventrata, 2009. Copyright 2009 Nicholas Judson. Pencil on paper drawing.
Nepenthes x ventrata, 2009.
Pencil on paper; 9" x 12".

The drawing above is of a plant in the Nepenthes genus. Nepenthes are carnivorous plants from the old world tropics. There are also carnivorous plants in the new world (e.g. Sarracenia species), with native species reaching up into North Carolina, Virginia, and beyond.

The evolution of carnivorous plants is interesting. One of their adaptations that has been selected for over the generations is the ability to get nitrogen not from the soil, but from insects and waste from insects and mammals that falls into their open pitchers or cups. Nitrogen is limiting in most environments, so there is a large selective pressure to obtain as much nitrogen as possible so that DNA, RNA, amino acids, and proteins can be made. Some plants (legumes, for example: peas, beans, clovers, alfalfa, soybeans) have evolved symbiotic relationships with bacteria that live in nodules in their roots to break apart atmospheric nitrogen and make it into a form that they can use (with the beneficial side effect of making more nitrogen than those legumes then consume, increasing the fertility of the soil). If mutations to your DNA over the previous countless generations have not conferred the ability to live in a mutually beneficial relationship with bacteria to get you nitrogen (for which you provide sugar from photosynthesis (6CO2 + 6H2O + light ---> C6H12O6 + 6O2 // ie, 6 carbon dioxides, 6 waters, and light gives you one sugar and 6 oxygens)), you must have some other advantage that can be selected for to enable you to compete effectively in a low nitrogen environment, otherwise you won't succeed.

The adaptation that has been selected for in carnivorous plants is a powerful one, and has been reached by plants in vastly different reaches of the world, a good example of convergent evolution. If you can absorb nitrogen from insects, it allows you to grow in areas that may be plentiful in water, but low in nitrogen -- many of the Sarracenia pitcher plants live in bogs and other low nutrient environments (more on carnivorous plants here).

The Nepenthes species drawn here, Nepenthes x ventrata, is a naturally occurring hybrid between two species of Nepenthes, N. alata and N. ventricosa. It is easy to grow in greenhouses and was one of the pitcher plants on display in the Jungle habitat at the US Botanic Garden in March 2009. The Botanic Garden has many more carnivorous plants and there are always some in the "Plant Adaptations" room, as an example of adaptations selected to obtain nitrogen from the environment. I chose to draw the cup of the plant -- the cup is a modified leaf and the tendril that supports the cup grows out of the end of another leaf (not shown). As seen in this drawing, the tendril often forms a loop, with the potential benefit of being able to wrap around another piece of plant to support the weight of the cup. The rim and top third of the inside cup is waxy, selected because it is slippery, which makes it more likely that insects will slip inside the cup. The middle third typically has hairs pointing downwards to impede the escape of struggling insects, and the bottom third has water and digestive enzymes so the plant can digest the insect and absorb nutrients.

I was inspired.

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