Gorgonia graminea, structure and habitat of, ii. 124, 125 Gorgonia verrucosa, structure, habitat, and mode of reproduction of, ii. 124 Gorgoniidæ, structure and mode of reproduction of, ii. 123, 124 the three natural groups of, ii. 124 Gracilaria compressa, i. 239 Gracilaria lichenoides, or Ceylon moss, i. 239 Gracilaria armata, mode of reproduction of, i. 230 Graminaceæ, structure and mode of reproduction of, i. 385, 386 Grammatophora serpentina, structure and development of, i. 197 Graphidei, characters of the order, i. 308 habitat of the, i. 309 Graphite, form of the crystals of, i. 15 natural, little or no porosity of, i. 112 porosity of artificial graphite, i. 112 Grasses, silica in the stalks and leaves of the, i. 17 Grasses, structure and mode of reproduction of, i. 385-387 Gravitation, force of, less powerful than that of molecular attraction, i. 109 Gravity, specific, of atoms, i. 100 Green dye, obtained from the buckthorn, i. 124 Griffithsia, structure, habitat, and organs of reproduction of, i. 233 Grimmiei, structure of the tribe, i. 329 Grinnelia americana, structure and mode of reproduction of, i. 230, 239 Gromia oviformis, structure of, ii. 26, 27 Gromiæ, structure of the genus, ii. 25-27 Guano, mauve dye obtained from, i. 125 Guernsey, richness of the iodine obtained from the sea-weeds of, i. 258 Guinea worm, structure and mode of reproduction of, ii. 147 Gums, formation of, i. 422 Gymnogramma rutæfolia, remarkable distribution of, i. 336 Gyrophora, structure of the genus, i. 308 H Hairs of plants, structure of, i. 411 Halichondria panicea, mode of propagation of, ii. 60, 61 Haliomma, structure of, ii. 21 Haliotis splendens, or ear-shell, structure of, ii. 234 Haliseris, structure of, i. 247 Halogens, spectra of the, i. 146 Halurus, structure and mode of propagation of, i. 233 Hare’s-foot fern, i. 351 Hart’s-tongue fern, caudex of, i. 340 structure, habitat, and fructification of, i. 351, 352 Heart, and organs representing it in the lower animals, ii. 4