針葉樹類の受粉機構  [in Japanese] Pollination mechanisms in conifers  [in Japanese]

針葉樹類での受粉機構は多様であるが, この多様性を各々の科で要約した。スギ科, ヒノキ科などの受粉には胚珠から分泌される受粉滴が必要である。これらの科の花粉は気嚢がなく, 受粉滴に沈む。胚珠が上を向いている場合は, 花粉は受粉滴の中を落下して直接珠心に届く。胚珠が下を向いている場合は, 受粉滴が体積を減少する際にこの液のメニスカスによって上に運ばれる。珠心の先端は親水性であり, 珠皮の内表面がクチクラで疎水性であるため, 受粉滴の液柱が珠孔道の中を珠心に向かって動くからである。マツ科のマツ属, トウヒ属またマキ科の多くの属でも受粉に受粉滴が関わる。これらの植物では胚珠が必ず下を向いていて, 気嚢で受粉滴に浮く花粉が珠孔の周りに浮揚する。花粉はその後, 液のメニスカスによって胚珠内に導き込まれるようである。マツ科のカラマツ属, トガサワラ属では珠皮の先端が胚珠内に陥入することで, 珠皮先端についていた花粉が胚珠内に運び込まれる。マツ科のツガ属, ナンヨウスギ科などでは苞葉に落ちた花粉がその位置で長い花粉管を珠心に向けて伸ばす。針葉樹類のおよそ半数の属でしか受粉機構が調べられておらず, 残りの属での研究が望まれている。今までの研究でも受粉滴の化学組成, 分泌と吸収(あるいは単に蒸散かもしれない)のメカニズム等は解明されておらず, 受粉機構と進化の関係についても研究が進んでいない。受粉機構は種子生産に直接関係するので森林の再生にも重要な意味を持つ。

Pollination mechanisms in conifers are diversified and generally correspond with taxonomic boundaries. In Tsuga, Hesperopeuce(Pinaceae), Saxegothaea(Podocarpaceae)and Araucariaceae pollen germinates outside the ovule and a long pollen tube penetrates the micropyle. In the remaining genera so far examined in conifers, pollen is captured by ovules and germinates in the micropylar canal. The pollen capture by ovules usually involves a pollination drop secreted from the ovule Non-saccate pollen in Cephalotaxaceae, Cupressaceae, Sciadopityaceae, Taxaceae and Taxodiaceae lands on a pollination drop. It simply sinks in the drop and reaches the nucellus if ovules are erect, or it moves upward carried by the meniscus of the drop if ovules are inverted. Since the nucellar apex is hydrophilic and the inner surface of the integument is hydrophobic, the meniscus of the pollintion drop moves upward when it recedes in the micropylar canal of the inverted ovules. Saccate pollen of Picea, Pinus(Pinaceae)and most genera of Podocarpaceae is also captured by a pollination drop. In these genera ovules are inverted, and pollen in contact with the drop floats upward to the micropyle and subsequently carried into the micropylar canal perhaps by the drop meniscus. In Cedrus rain water, instead of a pollination drop, is the main vehicle for pollen movement to the nucellus, and this may be also true in Picea and Pinus. In Larix and Pseudotsuga(Pinaceae)pollen adheres to the integument tip, and it is carried into the micropylar canal when the tip invaginates into the canal. Our knowledge on the pollination mechanisms in conifers thus far is based on about half of the extant genera. Clearly research efforts should be concentrated on the mechanisms in the remaining genera and poorly examined genera. Available literature suggests further intensive studies in such aspects as the chemical composition of the pollination drop, mechanisms of secretion and receding of the drop and the relation between pollen size and pollen movement in the micropylar canal. These studies would provide insight into evolutionary considerations and essential information for seed production which often limits reforestation programs in conifers.