h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I'm surprised that no-one has mentioned the class of soapy plant chemicals known as saponins. Saponins are chemically pretty similar to soaps, but have different components e.g. a saponin is a chemical bonding between a lipid (fat soluble) and a sugar (water soluble), whereas ordinary soap would seem to be a bond between a lipid and an alkaline substance (or glycerine in the case of Pears soap). In naming saponin molecules those responsible reached dizzy depths of imagination. The fat/lipid component is the "aglycone" (i.e. 'without sugar'). AND then, to cap it the sugar component is the "glycone". Truly staggering use of elan and inspiration, I feel.

You can tell if a plant contains saponins by the normal scientific method: THE FROTH TEST. Just mush up your plant part thoroughly (leaf, bark, root or whatever), place it in a clean glass jar and add water. Shake well. Sit back and observe the froth. If it is nice and firm, you have a plant with saponin in it. If it has a few bubbles that disperse quickly, then it is saponin free.

You can make a very useful wash for sensitive skins, hair, antique tapestries and etc. by using a solution of soapwort root (saponaria officianalis). You don't get much froth (compared to commercial shampoos), but you do get a very good gentle clean. Froth, and its strange association with "clean" or "effective" is more than somewhat overrated, actually.

Saponin rich plants are extensively used in herbal medicine systems, but an interesting use is found in traditional fish poisons. Fish poisons are found throughout the world in ethnobotany - South America, Australia etc. The plant part is pounded up thoroughly and dispersed throughout a waterhole (one method is to fill string bags with pounded plant material and swim around in the water until the fish begin to rise. Then you catch them with ease and have a nice fish dinner).

Why, then, does the plant kill the fish and have no effect on the swimmer? How can you safely eat fish that have been killed in this manner??

Well, the swimmer is protected by his/her skin. The fish are killed by direct contact between the membranes of their gills and the saponins. Saponins, if injected into the bloodstream, lyse cells - i.e. blow them up. You don't put saponin rich plant parts onto an open wound, for instance (or wash them with soap, for that matter). It is safe to eat the fish because the saponin is broken down in the digestive system, into the glycone and aglycone components, making it harmless (if not therapeutic, perhaps).

BUT - I'm certainly not recommending that anyone (repeat anyone) tries this on their little friend Hamlet the Goldfish just to see if it works. It will. And Hamlet will be no more.