牛樟之胚胎學

Abstract

牛樟(Cinnamomum kanehirae Hayata) 為台灣特有種，屬常綠大喬木。花兩性，1至3月開花，核果9、10月間成熟。 十二月中旬，花序原體出現。十二月下旬，雄蕊原體出現。花藥4室，向上瓣裂，藥瓣上具5(~6) 層細胞，分別為表皮層，內壁層，2(~3) 層中間層及營養層。營養層屬於類變形蟲型，營養層細胞具4核。小孢子母細胞於一月中下旬行減數分裂，胞質分裂屬於次第型，並產生兩側對稱形小孢子四分體，極少為四面體形或T字形。二月上旬花藥開裂，成熟花粉粒具二細胞，呈球形，無溝孔。 一月上旬，心皮原體出現。一月中旬，胚珠原體出現。胚珠倒生，兩層珠被，厚珠心，且於胚囊成熟時具2~5層細胞的珠心帽。一月中下旬，外珠被約3~5層細胞厚，內珠被約2~3層細胞厚，珠孔由內珠被形成。1個大孢子母細胞歷經減數分裂形成直線形的大孢子四分體，合點端大孢子具功能性。胚囊發育為蓼形，3個反足細胞於受精前便很快地退化。 胚乳發育為核型。胚之發育為柳葉菜型。成熟種子不具胚乳。子葉直立耳狀大形。

Cinnamomum kanehirae Hayata is an endemic species to Taiwan. It is a large evergreen tree. Hermaphrodite flowers are produced from January to March, and drupe mature during September to October. Inflorescence primordia occur in Mid-December. Anther primordia appear by late December. Anthers are 4-celled and open by uplifting valves. The anther walls of valves are five (to six) layers consisting of an epidermis, an endothecium, two (to three) middle layers, and a tapetum. The tapetum is of amoeboid type. Tapetal cells have up to four nuclei. Microspore mother cells undergo meiosis through successive cytokinesis from mid to late January and give rise to tetrads of microspores, which are mostly isobilaterally arranged and seldom tetrahedrally or in “T” shap. Anthers are dehiscent in early February. Mature pollen grains are 2-celled in early February, and they are spheroidal and inaperturate. Carpel primordia appear in early January. Ovule primordia appear in mid- January. Ovules are anatropous, bitegmic, crassinucellate and with 2~5 cell-layered nucellar cap while embryo sac is mature. From mid to late January, the outer integument is of 3~5 layers of cells in thickness, while the inner integument is composed of 2~3 layers of cells. The micropyle is formed by inner integument. A megaspore mother cell undergoes meiosis, resulting in a linear tetrad of megaspores. The megaspore at chalazal end is functional. The development of embryo sac is of the Polygonum type. Three antipodal cells degenerate soon before fertilization. Developing endosperm is of the nucellar type. The embryogeny is of the Onagrad type. Mature seed exalbumious; cotyledons straight, auricled and massive.