I don't wantta pick a fight with the book that was supposed to stop barroom brawls, drublic, but your's is not a correct answer. I dunno if it was Guiness or your source that made at least two errors. First there is no plant Genus "Aeschynome", there is an AeschynomEne hispida, an annual plant in the bean family, but it's native to the Caribbean, not southeast Asia, known as the Sensitive plant and other similar names. It is also called Pith Plant and similar, and that brings up the other error, which concerns the definition of "wood." " a material found as ... the stems of woody plants, especially trees... These perennial plants... are composed of cellulose and lignin based tissue. Plants that do not produce wood are called 'herbaceous'; this group of plants includes all annual plants " ( http://en.wikipedia.org/wiki/Wood ) ( A. hispida is an annual plant, by definition it does not have wood.) "Pith" is defined as "soft sponge-like central cylinder of the stems of plants " and "large, loosely packed ... cells. In trees the pith becomes much reduced as the woody tissue (lignin) replaces it." In other words it's either pith or wood. So, A. hispida loses on two counts. In practical usage "wood" also includes hard stiff plant material used in construction, that strictly speaking is not wood, bamboo, which is a grass, for example. Pith is certainly not hard or stiff. The "lightest wood," or least dense or with the lowest specific gravity is the familiar balsa, Ochroma pyramidale, of South America with a specific gravity of only 0.19 , 170 kg/cu.m. Perversely, balsa is classified as a hardwood, hardwood trees are angiosperms, ( broad leafed, usually deciduous, seed in some kind of fruit [oak and of course, balsa]) softwoods are gymnosperms ( needle leafed, usually evergreen, no fruit [pine trees]) Fool your friends, by a 4 foot balsa 2X4, break it with a powerful karate blow! Bullies won't pick on you any more. If anyone wants to argue, let's drink us some Guiness, you get a 2X4 of the hardwood balsa and I'll just use a soft one of pine and we'll duke it out lumberjack style, with frequent pith breaks due to the Guiness. ( My edit, [4/26 10:50 CDT] comments on comments by drublic and Glenn, really) " Do some research before you rate." Glenn, I guess you didn't see my "new answer" before you rated. Just correcting the misspelling from the original source doesn't make it correct. I found the same info, word for word, "The Guiness Book...weigh 2.75 lbs." including the misspelling on at least a half dozen sites. I do not know the original on- line source of the cut and pastes. I can't find my hard book so I do not know if it is the source of that error. Adding other links giving the same info does not make that source correct, it takes a real stretch of the definition to classify A. hispida as "wood." ( And I wood have sworn that your new link to the Canadian Marquetry Page was one of those I saw with the word for word quotes, but I guess not. As to the links to " Balsa is only about the 3rd lightest wood," or the fourth according to the second link, I guess fourth is "about 3rd." I see two caveats, the first is rather quibbling, I'll admit. Nether gives a source for the info or lists what the 2 or 3 lighter woods are. Does the original source consider A.hispida as the lightest wood? I've given my objections to A. hispida as "wood." My other objection concerns a mutually agreeable definition of "wood", for example, drublic's use of "COMMERCIAL WOOD" , my reference to bamboo, various references to "practical," "common," etc. http://www.onerateads.com/rc50h-b.htm says ..." the woods lighter than balsa are terribly weak and unsuitable for any practical use. The very lightest varieties don't really resemble wood at all, as we commonly think of it, but are more like a tree-like vegetable that grows in rings, similar in texture to an onion." Technically and scientifically these onion rings may be wood, but then so is a single strand of lignin, but 29 online dictionaries and five hard ones I have here all use the word "hard" in the definition. Practically speaking, and despite my degree in biology, I just can't consider something with the texture of an onion as being "wood" any more than I can an annual bean. At least one on-line source and two of my old text books distinctly mention perennial plants in the definition. I have grown some awfully "woody" onions myself, and radishes and carrots too. They are woody because of an overabundance of lignin, are we to consider them as being wood? What about the tumble weed (Genus Salsola)? Another annual with a pithy core but a very hard and tough lignin outer shell and a practical use as temporary fencing when dried. Most also say "lignified substance under the bark of trees," which lets out bamboo, but when it is used as structural or furniture material it is referred to as "wood." Technically speaking I can accept A. hispida as the lightest "wood", at SG of .044 and 2.75 lb/ft^3. Scientifically I gotta reject it as an annual. Practically I gotta accept balsa at SG 0.19 and 4 lb/ft^3 Incidentally, I chose 4lb from drub's and Glenn's links and not Glenn's 8, I've determined specific gravities of wood in the lab myself and the samples are kept in a drying kiln until they just don't get any lighter. (I'm out of Guiness but I've got some Shiner, let's all have one and talk about somethin' non-contreversial like politics)

From the previous answers by notmrjohn and drublic, both of whom provide some good information, it depends on how you define "wood." The types of "wood" that are lighter than balsa would not be considered wood by your average guy on the street. They are sort of like a soft, dry spongy substance that you can squish between your fingers. These substances might or might not be true woods from a botanist's standpoint but an average person would not recognize them as wood. __________________________________________________ More info from http://www.greathobbies.com/document/tech_balsa_1.html Is Balsa the lightest wood in the world? No! Most people are surprised to hear that botanically, balsa wood is only about the third or fourth lightest wood in the world. However, all the woods which are lighter than balsa are terribly weak and unsuitable for any practical use. The very lightest varieties don't really resemble wood at all, as we commonly think of it, but are more like a tree-like vegetable that grows in rings, similar in texture to an onion. It is not until balsa is reached that there is any sign of real strength combined with lightness. __________________________________________________ So, though Balsa is not technically the lightest wood, it is the lightest type of what we commonly think of as wood.