Leaf - (English spelling) leaf

Japanese: 葉 - は（英語表記）leaf

A leaf is one of the plant's organs. It is usually a flat, green organ that is regularly arranged around the stem and performs photosynthesis. However, there are various types of leaves, including those that do not have the function of photosynthesis and those that are not flat.

[Hara Hiroshi]

Ordinary leaves

(1) Leaf blade, petiole, and stipules A green leaf that performs photosynthesis is called a common leaf. A common leaf has three distinct parts: the blade, petiole, and stipule. A leaf that has all three parts is called a complete leaf, and a leaf that lacks one or two of these parts is called an incomplete leaf. The leaf blade is usually flat and easily absorbs sunlight, and is the part that actively performs photosynthesis, which is the leaf's main function. The petiole connects the stem and the leaf blade, and during the growth process, this part twists appropriately, and also functions to orient the leaf blade in the direction of the sunlight. Stipules are not found in ferns or gymnosperms, but are found in many angiosperms, especially dicotyledons. Usually, there is a pair of stipules at the base of the leaf, and many of them are thought to have the role of protecting the young leaf blade. In addition, the part at the base of the leaf that corresponds to the stipule becomes the leaf sheath, and the relationship between the leaf sheath and the stipule is sometimes questioned.

(2) Simple and Compound Leaves Leaves can be classified as simple or compound based on the shape of the leaf blade. A simple leaf is one whose blade consists of one leaf-like part, while a compound leaf is one whose blade is divided into multiple leaf-like parts, i.e., leaflets. Generally, the part that runs from the petiole through the center of the leaf blade to the tip of the leaf is called the rachis. Among compound leaves, those with multiple leaflets arranged on either side of the rachis are called pinnately compound leaves. Of these, those with a leaflet at the tip are called odd-pinnately compound leaves, and those without a leaflet at the tip are called even-pinnately compound leaves. Also, those with three or more leaflets radiating from the tip of the petiole are called palmately compound leaves.

A leaflet of a compound leaf can look very similar to a leaflet of a simple leaf. In particular, odd-pinnate compound leaves such as those of wisteria and rose resemble simple leaves that grow on a single branch. However, the distinction between a simple leaf and a compound leaflet can be made by looking at the axillary buds. Axillary buds grow in the axils of a single leaf, and leaflets and simple leaves can be distinguished by the location of the axillary buds and differences in the structure of the petiole and stem.

The leaf blade of a simple leaf and the leaflet blade of a leaflet can have various shapes, such as linear, needle-shaped, circular, or oval. In addition, there are leaves with serrated edges, notches, and leaves without serrated edges (called entire edges). When a single individual has leaves with significantly different shapes and sizes, they are called heteromorphic leaves.

(3) Leaf veins Leaves have veins that help move materials within the leaf. Leaf veins are broadly classified as follows. When the veins form a net, from the main veins to the relatively thin veins, they are called reticulate venation. When the main veins are pinnate, they are called pinnate venation, and when the main veins are palmately divided, they are called palmate venation. When the main veins are parallel, they are called parallel venation, and when the veins repeatedly divide into two, they are called bifurcated venation. Reticulate venation is common in dicotyledonous plants, while parallel venation is common in monocotyledonous plants. Bifurcated venation is also seen in many ferns and the gymnosperm ginkgo.

(4) Leaf Tissue Since leaves usually perform photosynthesis, the mesophyll (leaf tissue) that contains the chloroplasts that perform photosynthesis is prominent in the tissue of the leaf. In most cases, when a leaf is cut in cross section, the upper epidermis is at the top, followed by palisade tissue (a type of mesophyll tissue) below that, spongy tissue (also a type of mesophyll tissue), and finally the lower epidermis at the bottom. There are stomata in various places on the epidermis, surrounded by two guard cells with special shapes. Stomata are more common on the underside of leaves. The stomata open and close due to the action of the guard cells, and gas exchange between the plant body and the outside world, such as photosynthesis, respiration, and transpiration, takes place. There are abundant intercellular spaces (air spaces) between the cells of the mesophyll tissue, and the cells of the leaf tissue can exchange gas with the outside world through these spaces and stomata.

[Hara Hiroshi]

Leaves other than normal leaves

In addition to ordinary leaves, there are also cotyledons, which are the first leaves of the plant body, hypoemergent leaves that grow relatively low on the shoot, and hyperemergent leaves that grow higher on the shoot. The scale leaves that encase the buds are hypoemergent leaves, while the bracts that hold the flower in the leaf axils are hyperemergent leaves. The sepals, petals, stamens, and carpels that are components of flowers can also be interpreted as leaf-like. In this way, ordinary leaves and other leaf-like things are sometimes collectively called leaf-like organs. The needles of cacti are considered to be morphologically equivalent to the leaves of short branches, and are therefore treated as leaf-like organs and called leaf needles. The needles of barberry are also leaf needles. However, the needles of black locust correspond to stipules, and the tendrils of peas correspond to parts of the leaf blade.

[Hara Hiroshi]

Leaf life span

The young leaves are rolled up in various shapes within the bud (called the juvenile leaf stage) depending on the species, but usually become flat as the bud unfolds. The lifespan of the normal leaf after unfolding is from spring until the leaves turn red in autumn for deciduous trees, but varies from one year to several years for evergreen trees. In some species of the pine genus, needles have been reported to last nearly 40 years, and the exotic gymnosperm Welwitschia, which grows in the deserts of Africa, has a pair of leaves that continue to grow throughout its life and are said to live for over 1000 years.

[Hara Hiroshi]

Leaf Structure
©Shogakukan ">

Leaf Structure

Leaf Morphology
©Shogakukan ">

Leaf Morphology

How the leaves grow
©Shogakukan ">

How the leaves grow

Vein type
©Shogakukan ">

Vein type

Leaf tissue and gas exchange
©Takashi Aoki

Leaf tissue and gas exchange

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

植物の器官の一つ。普通は茎の周りに規則的に配列し、光合成を行う扁平(へんぺい)な形をもつ緑色の器官である。しかし、なかには光合成の機能をもたない葉、扁平ではない葉などさまざまなものがある。

［原　襄］

普通葉

(1)葉身・葉柄(ようへい)・托葉(たくよう)　光合成を行う緑色の葉を普通葉とよぶ。普通葉には、葉身と葉柄と托葉の三つの部分が区別されるが、このすべての部分を備えている葉を完全葉、このうちの一つまたは二つの部分を欠く葉を不完全葉とよぶ。葉身は、普通、平面的な形で日光を受けやすく、葉の主要な働きである光合成を活発に行う部分である。葉柄は茎と葉身を連絡する部分であるとともに、この部分がその成長過程で適当にねじれて、葉身を日光のくる方向に向ける働きももつ。托葉はシダ植物や裸子植物にはなく、被子植物のうち、とくに双子葉植物に多くみられる。普通、托葉は葉の基部に1対あり、若い葉身を保護する役割をもつと考えられるものが多い。また、葉の基部の托葉に相当する部分が葉鞘(ようしょう)となるものも多く、葉鞘と托葉との関係が問題とされることもある。

(2)単葉と複葉　葉は、葉身の部分の形から、単葉と複葉とに区別することができる。単葉とは葉身が一つの葉状の部分からなる葉であり、複葉とは葉身が複数の葉状の部分、すなわち小葉に分かれている葉をいう。一般に葉柄から葉身の中央を通って葉の先端に至る部分を葉軸というが、複葉のうち、葉軸を挟んで複数の小葉が左右に並ぶものを羽状複葉といい、このうち、先端に小葉のつくものを奇数羽状複葉、先端に小葉がないものを偶数羽状複葉とよぶ。また、葉柄の先端に小葉が放射状に三つ以上つくものを掌状複葉とよぶ。

　複葉の小葉の1枚と単葉の1枚とはよく似ている場合がある。とくにフジ、バラなどの奇数羽状複葉は1本の枝につく単葉と似ている。しかし、単葉であるか、複葉の小葉であるかの区別は、腋芽(えきが)によって知ることができる。腋芽は1枚の葉の葉腋につくものであり、小葉と単葉は、腋芽のつく位置、葉柄と茎との組織の相違などによって区別できる。

　単葉の葉身、小葉の小葉身には線形、針形、円形、卵形などさまざまな形がある。また、葉の周縁に鋸歯(きょし)（ぎざぎざ）のあるもの、欠刻（切れ込み）のあるもの、鋸歯も欠刻もないもの（全縁とよばれる）などがある。なお、一つの個体に形や大きさの著しく異なる葉があるとき、これを異形葉とよぶ。

(3)葉脈　葉には、葉の中における物質の移動に役だつ葉脈がある。葉脈は次のように大別される。おもな葉脈から比較的細い葉脈に至るまで葉脈が網目をつくる場合を網状脈とよび、このうち、おもな脈が羽状となる場合を羽状脈、おもな脈が掌状に分かれる場合を掌状脈とよぶ。また、おもな葉脈が平行となる場合を平行脈、葉脈が二又に分かれることを繰り返す場合を二又脈とよぶ。網状脈は双子葉植物に多く、平行脈は単子葉植物に多い。また、二又脈は、多くのシダ類と裸子植物のイチョウにみられる。

(4)葉の組織　普通葉が光合成を行うことから、普通葉の組織には、光合成を行う葉緑体を顕著にもつ葉肉（葉肉組織）が目だつ。もっとも普通の場合、葉の横断面をつくってみると、いちばん上に上側の表皮、その下に葉肉組織の一つである柵状組織(さくじょうそしき)、ついで同じく葉肉組織の海綿状組織、そしていちばん下に下側の表皮がある。表皮のところどころには気孔があり、特別の形をもった二つの孔辺細胞に囲まれている。気孔は葉の下側に多い。孔辺細胞の働きによって気孔が開閉し、光合成、呼吸、蒸散といった植物体内と外界との間のガス交換が行われる。葉肉組織の細胞間には、豊富な細胞間隙(かんげき)（空気間隙）があり、葉の組織の細胞はこの間隙と気孔を通して外界との気体の交換を行うことができる。

［原　襄］

普通葉以外の葉

葉には、普通葉のほかに、植物体の最初の葉である子葉、シュート（苗条(びょうじょう)）の比較的下につく低出葉、シュートの上のほうにつく高出葉がある。芽を包む鱗片(りんぺん)葉は低出葉の一つであり、花を葉腋に抱く包葉は高出葉である。花の構成要素である萼片(がくへん)、花弁、雄しべ、心皮も葉に類するものと解釈することができる。このように、普通葉をはじめとして葉に類するものをまとめて葉的器官とよぶことがある。サボテン類の針は形態学的には短枝の葉に相当するとみられるところから、葉的器官の一つとして扱われ、葉針とよばれる。メギの針も葉針である。しかし、ニセアカシアの針は托葉に相当するものであり、エンドウの巻きひげは葉身の一部に相当する。

［原　襄］

葉の寿命

幼い葉は、芽の中で、種類によってさまざまな形に巻いている（幼葉態という）が、芽の展開に伴って平面的な形となるのが普通である。展開のあとの普通葉の寿命は、落葉樹であれば春から秋の紅葉までの期間であるが、常緑樹では1年から数年のものまでとさまざまである。マツ属のある種では、針状葉が40年近くも生きていると報告されているし、アフリカの砂漠に生育する風変わりな裸子植物であるウェルウィッチアは終生伸び続ける1対の葉をもち、しかも、それが1000年を超える寿命をもつといわれている。

［原　襄］

[参照項目] | 異形葉 | ウェルウィッチア | 紅葉 | 葉肉 | 葉脈

葉の構造

葉の形態

葉のつき方

葉脈のタイプ

葉の組織とガス交換

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