Epidermis.—Outer layer of cells, which forms a skin, in the multicellular plants. See fig. 21, p. 54. Fruit.—Essentially consisting of a seed or seeds, enclosed in some surrounding tissues, which may be only those of the carpel, or may also be other parts of the flower fused to it. Thus a peapod is a _fruit_, containing the peas, which are seeds. Gannister.—A very hard, gritty rock found below some coal seams. See p. 25. Genus.—A small group within a family which includes all the plants very like each other, to which are all given the same “surname”; e.g. _Pinus montana_, _Pinus sylvestris_, _Pinus Pinaster_, &c. &c., are all members of the genus _Pinus_, and would be called “pine trees” in general (see “Species”). Hyphæ.—The delicate elongated cells of Fungi. Molecule.—The group of chemical elements, in a definite proportion, which is the basis of any compound substance; _e.g._ two atoms of hydrogen and one atom of oxygen form a molecule of water, H_2O. A lime carbonate molecule (see definition of “Carbonate”) is represented as CaCO_3. Monostelic.—A type of stem that contains only one stele. Morphology.—The study of the features of plants, their shapes and relations, and the theories regarding the origin of the organs. Nucellus.—The tissue in a Gymnosperm seed in which the large “spore” develops. See figs. 55 and 56, p. 76. Nucleus.—The more compact mass of protoplasm in the centre of each living cell, which controls its growth and division. See fig. 17, _n._ Palæobotany.—The study of fossil plants. Palæontology.—The study of fossil organisms, both plants and animals. Petiole.—The stalk of a leaf, which attaches it to the stem. Phloem.—Commonly called “bast”. The elongated vessel-like cells which conduct the manufactured food. See p. 57. Pollen Chamber.—The cavity inside a Gymnosperm seed in which the pollen grains rest for some time before giving out the male cells which fertilize the egg-cell in the seed. See p. 76. Polystelic.—A type of stem that appears, in any transverse section, to contain several steles. See note on the use of the word on p. 63. Protoplasm.—The colourless, constantly moving mass of finely granulated, jelly-like substance, which is the essentially living part of both plants and animals. Rock.—Used by a geologist for all kinds of earth layers. Clay, and even gravel, are “rocks” in a geological sense. Roof, of a coal seam. The layers of rock—usually shale, limestone, or sandstone—which lie just above the coal. See p. 24. Sclerenchyma.—Cells with very thick walls, specially modified for strengthening the tissues. See fig. 28, p. 56. Seed.—Essentially consisting of a young embryo and the tissues round it, which are enclosed in a double coat. See definition of “Fruit”. Shale.—A fine-grained soft rock, formed of dried and pressed mud or silt, which tends to split into thin sheets, on the surface of which fossils are often found.