When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and are either broken off or fall off. Subsequent growth of wood may completely conceal the stubs which will however remain as knots. No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle. Consequently, the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the inner heartwood. Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties, it follows that a given piece of sapwood, because of its position in the tree, may well be stronger than a piece of heartwood from the same tree.
What usually covers the whole trunk of a very young tree?
limbs

Tristan da Cunha /trstn d kunj/, colloquially Tristan, is both a remote group of volcanic islands in the south Atlantic Ocean and the main island of that group. It is the most remote inhabited archipelago in the world, lying 2,000 kilometres (1,200 mi) from the nearest inhabited land, Saint Helena, 2,400 kilometres (1,500 mi) from the nearest continental land, South Africa, and 3,360 kilometres (2,090 mi) from South America. The territory consists of the main island, also named Tristan da Cunha, which has a northsouth length of 11.27 kilometres (7.00 mi) and has an area of 98 square kilometres (38 sq mi), along with the smaller, uninhabited Nightingale Islands and the wildlife reserves of Inaccessible and Gough Islands.
what type of island group are the islands?
archipelago

Drinking water supply and sanitation in Egypt is characterised by both achievements and challenges. Among the achievements are an increase of piped water supply between 1990 and 2010 from 89% to 100% in urban areas and from 39% to 93% in rural areas despite rapid population growth, the elimination of open defecation in rural areas during the same period, and in general a relatively high level of investment in infrastructure. Access to an improved water source in Egypt is now practically universal with a rate of 99%. About one half of the population is connected to sanitary sewers.
What basic part of civilization has been characterized by achievement and challenges?
Drinking water supply and sanitation