Because rhubarb stems contain a lot of oxalic acid, and they’re purple. And clover roots are also purple, and develop purple specks in the leaves as they age and their oxalic acid content increases. And dandelions are the same, but not oxalis plants as far as I can tell… though they do have purple root bulbs…
Yes, I know they don’t contain oxalic acid as such, but oxalates.
Yes, I know there are
other reasons a plant might be purple.
I dunno why this popped into my head, but is there a correlation?
Oxalis not purple ? Or did I misunderstand ?
Ah… right. The kind we get here is yellow with green leaves.
I think it’s
this kind, but if not it at least looks like it. I didn’t know they came with different coloured leaves but it only adds to my hunch.
Oxalates are found in rhubarb not so much the stem. leaves
Oxalates are contained in all parts of rhubarb plants, especially in the green leaves. There is some evidence that anthraquinone glycosides are also present and may be partly responsible. It is not clear as to the exact source of poisoning from rhubarb, possibly a result of both compounds. The stalks contain low levels of oxalates, so this does not cause problems.
Anthocyanins, are the major purple pigments in plants. Most oxalic acid salts are white.
Interesting question. In plants where purple is not a normal color, purple stems/petioles can be a sign of phosphorus deficiency. I am not sure if this means anything in relation to the OP, but hey.
Hmm… Poke is another semitoxic plant with great purple stems and oxalic acid. Beets have oxalic acid.
But so do cabbage, potatoes and peas.
Pokeberry’s got other toxins as well:
Several toxins have been identified in species of Phytolacca, usually concentrated in the roots, berries and seeds. These poisons include an alkaloid (phytolaccine), a resin (phytolaccatoxin), and a saponin (phytolaccigenin). According to W.H. Lewis and M.P.F. Elvin-Lewis (Medical Botany, 1977), the most serious health hazard from Phytolacca comes from a very toxic plant protein called a lectin. Lectins can cause red blood cells to clump together (agglutinate) and may stimulate abnormal cell division in quiescent B and T-lymphocytes.
The Aubergine that ate Rangoon, does not appear to contain significant Oxalic Acids according to my quick web search.
But the very green Spinach does as does
Foods that contain significant concentrations of oxalic acid include (in decreasing order): buckwheat, star fruit (carambola), black pepper, parsley, poppy seed, rhubarb, spinach, chard, beets, banana, cocoa, chocolate, most nuts, most berries, and beans. In the case of rhubarb the only edible portion is the stalk because the root and leaves contain dangerously high concetrations of oxalic acid.