Inflorescence - Kajo

A group of multiple flowers is called an inflorescence. The arrangement of flowers in an inflorescence has a certain pattern depending on the type of plant, which is called the inflorescence type, but in general, the inflorescence type is also simply called the inflorescence. If there is only one flower at the end of the stem, it does not form an inflorescence, but when this is considered a type of inflorescence type, it is called a uniterminal inflorescence.

There are various types of inflorescences, but broadly speaking, they can be divided into simple and compound inflorescences, and simple inflorescences can be broadly divided into panicles and cymes. The essence of a flower is considered to be a specialized receptacle at the end of a stem, with specialized leaves such as perianths, stamens, and carpels attached, in other words, a specialized shoot (also called a shoot, which is a stem and the leaves attached to it considered as a single entity), so the inflorescence type is closely related to the branched type.

A panicle inflorescence is based on a single axis, with multiple branches coming out of a single axis each producing a flower. If there is a distance between the branching points where the branches come out of the axis and each flower has a stalk, it is called a raceme (such as wisteria and willow herb). A similar type, but with no stalks, is called a spike (such as plantain). Spikes with thick axes are called spadix, and are often found in the Araceae family. The spikes of the Poaceae family are compound inflorescences as a whole, but the spikelets, which are their constituent units, have the shape of a spike. If a type resembles a raceme, but the relationship between the length of each pedicel and the distance between the attachment points of the pedicels results in all the flowers being arranged in a plane or hemisphere, it is called a corymb. Sometimes rapeseed is given as an example of this, but this is not necessarily appropriate. Even if the inflorescence is a corymb at the time of flowering, the axis will later elongate, increasing the distance between the attachment points of the pedicels, and the inflorescence, i.e., the arrangement of the fruits, will become racemose. Examples of typical corymbs are Japanese quince and pear. An umbel is an inflorescence in which there is no distance between the attachment points of the pedicels, and several stalked flowers radiate from the end of the axis. This is seen in Japanese ginseng and Ardisia arborescens. An inflorescence in which a group of flowers without stalks radiates from the end of the axis is called a head inflorescence. In the Asteraceae family, most inflorescences that look like a single flower are head inflorescences. They are also called heads for short. For many flowers to grow at the end of the axis, there must be a certain amount of surface area, and the surface area of ​​the tip of the axis is large, although the shape varies depending on the species, such as cup-shaped, spherical, or conical.

In the inflorescences of the panicles described so far, the flowers open in order from the closest to the base of the axis, that is, the flowers open in an acropetal manner, and such inflorescences are called acropetal inflorescences. If viewed from above, the acropetal order can also be said to be centripetal, so they are also called centripetal inflorescences. However, there are also cases where flower buds are formed acropetally and then the order is reversed and they flower basilicately, so it is problematic to place too much emphasis on the order of flowering. The term indeterminate inflorescences is often used synonymously with acropetal inflorescences, but this term refers to the absence of flowers at the apex of the axis rather than the order of flowering. Racemes and corymbs with flowers at the apex are seen in plants such as Iris japonica and Ipomoea batatas, but these are acropetal and determinate inflorescences.

A cyme is an inflorescence formed by repeating the process of the apex of the axis becoming a flower, the axis stopping its growth, and the apex of a lateral branch becoming the next flower. Since the flower is at the apex, it is a determinate inflorescence, and since flowers emerge one after another from below, it is also called a basal cyme or efferent inflorescence. Depending on the number of branches that emerge from one branching, cymes are classified as solitary cymes (simple cymes), bipartite cymes (bifurcated cymes), and multiple cymes (multiple cymes). Each is closely related to alternate, opposite, and whorl leaf arrangements. Solitary cymes include fan-shaped inflorescences, in which the second and subsequent branches always appear on the adaxial side, sickle-shaped inflorescences, in which the flowers always appear on the abaxial side, snail-shaped inflorescences that grow laterally in a fixed direction, either to the right or left, and scorpion-shaped inflorescences that grow laterally alternately to the left and right. There are not many examples of solitary cymes in our daily lives, but examples include the snail-shaped inflorescences of Day Sedge and the scorpion-shaped inflorescences of the Violaceae family. In diplophyllaceae, the direction of the branches usually changes by 90 degrees each time. There are many examples in the Caryophyllaceae and Celastraceae families. When diplophyllaceae with no elongated pedicels grow in the axils of opposite leaves, the flowers appear to be arranged in a whorl around the nodes, and this is called a whorled inflorescence. A dichotomous cyme with no elongated flower stalk that grows at the tip of a stem is called a cyme. It is similar to a capitulum, but the flowering order is different. Examples can be found in the Dipodium and Cornaceae families. There are no suitable examples of polychotomous cymes, but many Euphorbiaceae plants have cup-shaped inflorescences in this format.

An inflorescence in which the inflorescence type is duplicated two or more times is called a compound inflorescence. A compound inflorescence with the same type repeated is called a homotypic compound inflorescence, and a heterotypic compound inflorescence with different types in parts and the whole. In the case of homotypic inflorescences, if the racemes are gathered in a raceme, the word "compound" is added to the inflorescence type name, such as a compound raceme. The Umbelaceae family has been called the Umbel family because of the characteristics of the inflorescence, but the majority of the Umbelaceae family are not simple umbels but compound umbels. In the case of heterotypic compound inflorescences, the name of the terminal type is followed by the name of the overall type, such as a capitulum if the capitulum is gathered in a raceme. A panicle is a general term for a large number of flowers gathered together to form an approximately cone shape as a whole, but in reality it is often a compound raceme. A panicle in which cymes are gathered in a raceme or compound raceme is called a dense panicle.

Some inflorescence types are unique to certain plant groups. The cup-shaped inflorescence found in the Euphorbiaceae family has one female flower with a single pistil and several male flowers with a single stamen in a urn-shaped container, and is considered a special type of cyme. The cryptocephalic inflorescence found in the Ficus genus in the Mulberry family is also considered a special type of cyme, with many tiny flowers on the inner wall of a fleshy urn-shaped container. A cluster of male flowers that have no or inconspicuous perianths and form a long, slender, usually drooping spike is called a catkin, and is found in the Salicaceae, Juglandaceae, Fagaceae, and Betulaceae families. Plants that produce catkins are sometimes collectively called the catkin group.

[Fukuda Taiji]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

複数の花が集団をなしているものを花序という。花序における花の配列様式には植物の種類に応じて一定の型があり、これを花序型というが、一般には花序型を単に花序ともいう。茎の先端にただ1個の花をつけただけの場合は花序をなさないわけであるが、これを花序型の一種とみるときは単頂(たんちょう)花序とよぶ。

　花序を分類するとさまざまな型が区別できるが、大きくまとめれば単花序と複花序になり、単花序は総穂(そうすい)花序と集散(しゅうさん)花序に大別することができる。花の本質は、茎の先が特殊化した花托(かたく)に葉が特殊化した花被片(かひへん)や雄しべや心皮がついたもの、すなわち特殊な苗条(びょうじょう)（シュートともいい、茎とそれについている葉とを一つのまとまりとみたもの）であると考えられるので、花序型は分枝型と関連が深い。

　総穂花序は単軸(たんじく)分枝に基づくもので、単一の軸から出た複数の枝がそれぞれ花となっている。軸から枝が出る分岐点どうしの間に距離があり、個々の花に柄があれば総状花序(そうじょうかじょ)という（フジ、ヤナギランなど）。これに似るが個々の花に柄がないものは穂状花序という（オオバコなど）。穂状花序のうち軸が肉太のものを肉穂花序(にくすいかじょ)といい、サトイモ科に例が多い。イネ科の穂は全体としては複花序であるが、その構成単位である小穂(しょうすい)には穂状花序の形式がみられる。総状花序に似るが、各花柄の長さと花柄付着点間の距離との関係ですべての花がほぼ一平面上ないし半球面上に並ぶような形ならば散房花序(さんぼうかじょ)という。アブラナをこの例にあげることがあるが、かならずしも適切ではない。開花時には散房花序であっても、のちに軸が伸長して花柄付着点どうしの距離が増し、果序すなわち果実の配列は総状となるからである。典型的な散房花序の例にはコデマリ、ナシなどのほうがよい。花柄付着点間の距離がなく、軸の先端から柄のある花がいくつか放射状に出ている形を散形花序(さんけいかじょ)という。トチバニンジン、ウコギなどにみられる。軸の先端に柄のない花が集合した形式を頭状(とうじょう)花序という。キク科で1個の花のようにみえるのはたいてい頭状花序である。略して頭花ともいう。軸の先端に多数の花がつくにあたっては、その場所にそれなりの面積がなければならないわけであるが、軸の先端は杯(さかずき)状、球状、円錐(えんすい)状など種類に応じて形は異なっても、表面積が広がっている。

　これまで述べてきた総穂花序においては、軸の基部に近い花から順に開く、すなわち求頂(きゅうちょう)的に開花が進むのが原則で、このような花序を求頂花序という。見方を変えて上から見れば、求頂的順序は求心的ともいえるので、求心花序ともいう。ただし、花芽(かが)形成が求頂的に行われたのちに順位が逆転して求基(きゅうき)的に開花すると思われる例もあり、開花順序を重視しすぎるのは問題である。無限花序という語も求頂花序と同義に用いられることが多いが、この語は開花順序よりも軸の頂端に花をつけないことを示すものである。頂端に花をつけた総状花序や散房花序がシャガやカジイチゴなどにみられるが、これらは求頂花序であり、かつ有限花序である。

　軸の頂端が花となってその軸の成長が止まり、側枝の頂端が次の花となるのを反復してできた花序を集散花序という。頂端が花となるので有限花序であり、次々に下方から花が出るので求基花序または遠心花序ともよばれる。集散花序は1回の分枝で出る枝の数によって、単出(たんしゅつ)集散花序（単散花序）、二出集散花序（岐散(きさん)花序）、多出(たしゅつ)集散花序（多散(たさん)花序）に分けられる。それぞれ、互生(ごせい)、対生(たいせい)、輪生(りんせい)の葉序と関係が深い。単出集散花序には、2回目以降の分枝がつねに向軸側に出る扇形花序(おうぎがたかじょ)、つねに背軸(はいじく)側に出る鎌形花序(かまがたかじょ)、右または左の一定方向に側生するかたつむり状花序、左右交互に側生するさそり状花序がある。単出集散花序の例は身近に多くはないが、ユウスゲのかたつむり状花序、ムラサキ科のさそり状花序などをあげることができる。二出集散花序においては、分枝の方向は毎回90度変わるのが普通である。ナデシコ科、ニシキギ科など実例は豊富にある。対生する葉の腋(えき)に花柄の伸びない二出集散花序がつくと、節の周囲に花が輪生状に並んでみえるので、これを輪散花序(りんさんかじょ)という。花柄の伸びない二出集散花序が茎の頂端にできたものを団散花序(だんさんかじょ)という。頭状花序に似るが、開花順序が異なる。マツムシソウ科、ミズキ科などに例がある。多出集散花序は適切な例がないが、トウダイグサ科には杯状花序をこの形式でつけるものが多い。

　花序型が2回以上重複している花序を複花序という。同一形式の反復したものを同型複花序、部分と全体とで形式の異なるものを異型複花序とよぶ。同型の場合は、総状花序が総状に集まっていれば複総状花序のように、花序型名の上に複をつけてよぶ。セリ科を花序の特徴から散形科とよんだことがあるが、セリ科の大部分にみられるのは単花序の散形花序ではなく複散形花序である。異形複花序の場合は、頭状花序が総状に集まっていれば頭状総状花序というなど、末端の形式の名の次に全体の形式の名をつけてよぶ。円錐花序は、多数の花が集まって全体としてほぼ円錐形をなしたものの総称でもあるが、実体は複総状花序であることが多い。集散花序が総状または複総状に集まった円錐花序を密錐花序(みっすいかじょ)という。

　花序型には、特定の植物群だけに固有のものもある。トウダイグサ科にみられる杯状花序（壺状(こじょう)花序）は、雌しべ1本からなる雌花1個と、雄しべ1本からなる雄花数個が壺(つぼ)形の器の中にあるもので、集散花序の特殊形と考えられる。クワ科イチジク属の隠頭(いんとう)花序も集散花序の特殊形と考えられ、多肉となった壺形の器の内壁に微小な花が多数ついている。花被がないかまたは目だたない雄花が集合して、細長く、通常は下垂する穂になったものを尾状花序(びじょうかじょ)といいヤナギ科、クルミ科、ブナ科、カバノキ科などにみられる。尾状花序をつける植物群をまとめて尾状花序群とよぶことがある。

［福田泰二］

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