It is of course impossible here to give any idea of the complexity of structure of our forest trees. A slice across the stem of a tree shows many different tissues with more or less technical names, bark and cambium, medullary rays, pith, and more or less specialised tissue; air-vessels, punctate vessels, woody fibres, liber fibres, scalariform vessels, and other more or less specialised tissues. Let us take a single leaf. The name is synonymous with anything very thin, so that we might well fancy that a leaf would consist of only one or two layers of cells. Far from it, the leaf is a highly complex structure. On the upper surface are a certain number of scattered hairs, while in the bud these are often numerous, long, silky, and serve to protect the young leaf, but the greater number fall off soon after the leaf expands. The hairs are seated on a layer of flattened cells--the skin or epidermis. Below this are one or more layers of "palisade cells," the function of which seems to be to regulate the quantity of light entering the leaf. Under these again is the "parenchyme," several layers of more or less rounded cells, leaving air spaces and passages between them. From place to place in the parenchyme run "fibro-vascular bundles," forming a sort of skeleton to the leaf, and comprising air-vessels on the upper side, rayed or dotted vessels with woody fibre below, and vessels of various kinds. The under surface of the leaf is formed by another layer of flattened cells, supporting generally more or less hairs, and some of them specially modified so as to leave minute openings or "stomata" leading into the air passages. These stomata are so small that there are millions on a single leaf, and on plants growing in dry countries, such as the Evergreen Oak, Oleander, etc., they are sunk in pits, and further protected by tufts of hair. The cells of the leaf again are themselves complex. They consist of a cell wall perforated by extremely minute orifices, of protoplasm, cell fluid, and numerous granules of "Chlorophyll," which give the leaf its green colour. While these are, stated very briefly, the essential parts of a leaf, the details differ in every species, while in the same species and even in the same plant, the leaves present minor differences according to the situation in which they grow. Since, then, there is so much complex structure in a single leaf, what must it be in a whole plant? There is a giant sea-weed (Macrocystis), which has been known to reach a length of 1000 feet, as also do some of the lianas of tropical forests. These, however, attain no great bulk, and the most gigantic specimens of the vegetable kingdom yet known are the Wellingtonia (Sequoia) gigantea, which grows to a height of 450 feet, and the Blue Gum (Eucalyptus) even to 480.