section of the annual rings, owing to the straight cut through the bent tree. The medullary rays are seen edgeways as fine lines in this section, whilst the annual layers form beautiful wavy and hearty grain. A plank cut so as to contain part of the centre pith of the tree as shown at F, in Fig. 126, would be least affected in breadth by shrinking.
Difference Between Exogenous and Endogenous Timber.—Exogens and endogens are very different in internal structure and in outward appearance. The exogens, as has been explained, increase in size by the addition of new material at the outside of the stem—just under the bark. They continue to increase in diameter as well as in height throughout their whole lifetime. This growth may be carried on continuously, as in the cactuses, or intermittently, by abrupt periodical advances and cessations, as in the forest trees. The hardest portion of the stem is towards the centre. The fibro-vascular bundles are "open" — that is, capable of further development. There is a distinct and separable bark, and usually a number of branches. The trunk and branches are frequently crooked. The leaves are articulated, and drop off neat or clean from the tree. The veins in the leaves ramify, forming an irregular network. The flowers, when present, have, as a rule, four or five sepals and petals, etc., or multiples of these. The seeds (except in conifers) split in two. The oak, apple, laburnum, and the wallflower are examples of exogens. Some exogens live to be more than a thousand years old. Endogens mainly increase in size by end growth. There is lateral distension for a time, but this soon ceases, and then the tree remains of nearly uniform diameter throughout its life. There are no annual rings—the growth being mostly continuous. The hardest portion of the stem is at the outside, where a false rind made up of broken leaf-ends, etc., is formed, but no bark. The fibro-vascular bundles become "limited," or "closed," after a certain period, after which they serve only to strengthen the stem. The trunk is straight, or nearly so, and seldom has any branches. If it does have any branches, as in bamboo, then these are straight too. At the top end, where the growing is taking place, the new leaves arise inside the old ones, and press them outwards and downwards as they grow. The old leaves eventually die, and hang like a ragged sheath around the stem. The leaves are parallel-veined. The flowers are mostly on the plan of three. The seed is entire: hence Monocotyledons. Few endogens live to be 300 years old. Nearly all the principal kinds belong to tropical or sub-tropical climates—examples are the palms, bamboos, grasses, and lilies. There are no endogenous trees indigenous to England, and it is believed that the only British endogenous shrub is the butcher's broom—Ruscus aculeatum.
Function of Sap.—The action of the sap may now be described in fuller detail. In the spring the roots absorb from the soil moisture, which, converted into sap, ascends through the cellular tubes to form the leaves. At the upper surface of the leaves the sap gives off moisture, absorbs carbon from the air, and becomes denser; after the leaves are full-grown, vegetation is suspended until the autumn, when the sap in its altered state descends, by the underside of the leaves, chiefly between the wood and the bark, where it deposits a layer of new wood (the annual ring for that year), a portion at the same time being absorbed by the bark. During this time the leaves drop off, the flow of sap then almost stops, and vegetation is at a standstill for the winter. With the next spring the operation recommences, so that after a year a distinct layer of wood is added to the tree. The above description refers to temperate climates, in which the circulation of sap stops during the winter; in tropical climates it stops during the dry season. Thus, as a rule, the age of the tree can be ascertained from the number of annual rings; but this is not always the case. Sometimes a recurrence of exceptionally warm or moist weather will produce a second ring in the same year.
Heartwood and Sapwood.—A young tree is almost all sapwood, but as it matures this is gradually changed into heartwood more rapidly than sapwood is added, and as the tree increases in age, the inner layers are
