Hardwood - Koyoju

The opposite of conifers, it is a dicotyledonous tree among angiosperms with large leaves. It is also called broadleaf tree. It originated in the tropics and spread to the northern dry regions as it acquired cold adaptations such as deciduous trees and dry adaptations such as sclerophyll trees from evergreen broadleaf trees. Unlike conifers, which are mainly wind-pollinated, many species have large flowers and have developed a method of attracting insects to pollinate. It is currently the most widely distributed and dominant plant on Earth. The wood of broadleaf trees is called hardwood, and is generally hard with fibers as its basic structure. The leaves have reticulate veins and no inner bark. They are divided into evergreen broadleaf trees and deciduous broadleaf trees, but evergreen trees are found in humid regions from the tropics to the warm temperate zone, deciduous trees that form raingreen forests in areas with a clear dry season, and sclerophyll trees are found in areas with a Mediterranean climate. Deciduous broadleaf trees are the main species in the temperate zones further north. In Japan, deciduous trees such as beech, maple, birch, Mongolian oak, and Konara oak are distributed in the cool temperate zone, while evergreen broad-leaved trees such as Machilus thunbergii, Castanopsis sieboldii, and Akagashi oak are distributed in the warm temperate zone. These evergreen trees have glossy leaves and are also called laurel trees.

[Masahiko Osawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

針葉樹の対語で、大きな葉をもつ被子植物のうちの双子葉類の樹木のこと。闊葉樹(かつようじゅ)ともいう。熱帯に起源し、常緑広葉樹から落葉性のような寒冷適応、硬葉性のような乾燥適応の獲得とともに北方乾燥地域に分布を広げた。風媒花を主とする針葉樹に対して、大形の花をもち、昆虫を誘引して受粉する様式が発達している種が多い。現在、地球上でもっとも広く分布し、優占している植物である。広葉樹の材は硬材とよばれ、繊維が基本的な組織となっており、一般に硬い。葉は網状脈で、内皮はみられない。常緑広葉樹と落葉広葉樹に分けられるが、熱帯から暖温帯までの湿潤地域は常緑樹、明瞭(めいりょう)な乾期をもつ地域では雨緑林を形成する落葉樹、地中海式気候の地域には硬葉樹が分布する。さらに北の温帯地方は落葉広葉樹が主体である。日本では冷温帯にブナ、カエデ類、カンバ類、ミズナラ、コナラなどの落葉樹、暖温帯にタブノキ、スダジイ、アカガシなどの常緑広葉樹が分布する。これらの常緑樹は葉面に光沢があり、照葉樹ともよばれる。

［大澤雅彦］

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