Endogenous woods are hardest and most compact at circumferences. The stems of palm trees are solid, but those of some of the grasses, particularly those that grow quickly, are hollow.[107] The tube or canal, when existing, is due to sluggishness on the part of the central pith, which, developing more slowly than the outer tissues, finally ruptures and disappears at the center. There are also more or less permanent joints or knots,[108] such as are made familiar by the canes and bamboos. The stems of Endogenous plants are seldom cut up into lumber, but are used in segments, or else entire, as for troughs or piles.[109] The use of Palm wood must be more or less [p184] limited to the neighborhoods in which such trees flourish, but it is probable that the Bamboo can be much more generally employed. The Endogens include numerous families and many thousand species.[110] The grasses, including wheat, rye, and Indian corn at the North and sugar-cane and bamboo at the South, belong to this group. Most Endogens are herbs; comparatively few furnish material for structural purposes. The Palms, including the palmetto, rattans, cane palms, and others, the Yucca, including the Joshua tree, Spanish bayonet, and others, and the Bamboos, representatives of the grasses, are thus useful. Endogens are also known as Monocotyledons. [p185] FOOTNOTES [Footnote 107: The Bamboo, which is a grass, is hollow, while the cornstalk, which is also a grass, is not.] [Footnote 108: The knots of endogens correspond to the nodes of exogens. Spaces between the nodes, known as internodes, mark the annual lengthening. Knots are places whence leaves have emerged.] [Footnote 109: Palm in marine work appears to repel the teredoe. This is probably because of the porous character of the wood. See "Marine Wood Borers," Trans. Am. Soc. C. E., Vol. XL, pages 195 and 204.] [Footnote 110: Bastin ("College Botany," p. 379) divides into about fifty natural orders distributed among seven divisions. Warming ("Systematic Botany," pp. 277, 278) divides into seven families corresponding with Bastin's seven divisions. A. Gray divides into twenty-one orders or families. Coulter ("Plants," p. 237) divides into forty families, including twenty thousand species.] [Illustration: PLATE 37. PALM (_Palmaceæ_) Cabbage Palmetto. (_Courtesy N. C. Geological Survey_.) Washington Palm (untrimmed). (_Messrs. Doubleday, Page & Co._). Washington Palm (trimmed). (_Los Angeles Chamber Commerce_.)] PALM. PALMACEÆ. This is one of the largest and most important orders of plants known to man. The one thousand[111] or more known species are distributed over the tropical and semitropical regions of the entire world. Only a few species, including the palmettos of the Gulf States and the fan palms of California are native in the United States.