台東蘇鐵之胚胎學

Abstract

中文摘要 台東蘇鐵(Cycas taitungensis)為雌雄異株。自孢子葉原體至胚珠成熟需時一年。 成熟之雄毬花為圓錐形，通常單生於雄株頂端。上具有945片小孢子葉，螺旋狀排列，每片小孢子葉背軸面著生數百個小孢子囊，每個小孢子囊柄上具有2~4個小孢子囊。 雄花器發育至成熟需時三個月。二月初，小孢子葉原體出現。至三月初，小孢子囊始源細胞出現於小孢子葉背軸面。四月初小孢子囊內造孢細胞行多次細胞分裂後產生數百個小孢子母細胞，之後小孢子母細胞即行減數分裂產生小孢子四分體。至四月中旬，小孢子四分體已各自分離形成單核小孢子，之後形成二核花粉粒。 四月下旬至五月中旬形成具三細胞之成熟花粉粒，分別為管細胞，生殖細胞，及原葉體細胞。至五月中旬之後成熟小孢子囊開裂花粉散出。 六月中位於珠心之花粉粒萌發。至七月底，珠心頂端之花粉管內具有一個體細胞，一個柄細胞，一個原葉細胞及一個管核。八月中授精，花粉管先端破裂釋出內含精細胞之物質於藏卵器腔。游動精子於藏卵器內與卵結合產生合子。 成熟之雌毬花由數百片大孢子葉組成。每片大孢子葉基部具有胚珠2~6枚。成熟胚珠具一層珠被。珠被間之空腔為珠孔道。珠被內為珠心，珠心內空腔為花粉腔。胚珠中央為大配子體。大配子體珠孔端具有藏卵器2~4個，每個藏卵器內具卵一枚。授精時各自授精且生長，但最後僅一個成胚。 雌花器起源於二月中，雌珠頂端產生雌毬花原體。三月初胚珠始原細胞出現於大孢子葉側面基部表皮層下方。至三月底胚珠原體開始隆起，產生珠被原體與珠心原體。隆起之珠被間自然形成珠孔道。四月上旬珠心中央產生造孢細胞。至四月中旬一枚造孢細胞發育成一大孢子母細胞。四月中旬大孢子母細胞行減數分裂形成大孢子四分體，其中三枚退化僅餘最靠合點端之功能性大孢子。其後功能性大孢子開始行核分裂後產生自由核。四月中珠心內形成花粉腔。五月底，雌配子體內之自由核數量估計達數千個，而雌配子體珠孔端產生藏卵器始原細胞。藏卵器始原細胞經分裂形成兩個初生頸細胞與一個中央細胞，六月初中央細胞再分裂形成一個卵及一個腹溝核。八月中卵授精。至八月底原胚開始發育。種子至隔年二月成熟，具子葉2枚。

Summary Cycas taitungensis is dioecism. Whole grows stage spending about one year. The male cone is conic shape. It mostly grows on the top of the plant lonely. It includes of several hundreds microsporophylls， arranging in twist shape. Several hundreds microsporangiums are gained on the dorsal site of microsporophyll. Each stipe grow up two to four microsporangiuns. Developing to mature form of male cone needs three months. Since early February, microsprophyte initial appear . In early March, microsporangium initial could be found on the dorsal site of microsporophyte. In early April, several times of mitosis of sporogenous cell in the microsporangium occur and several hundreds microspore mother-cells are bred. Microspore mother-cell goes on meiosis to develop tetrad. In middle April, microspore tetrads disparts to single nucleus microspore, than growth to two-nucleus pollen grain. From middle April to middle May, each mature pollen grain has three cells insite. There are tube cell, generative cell and prothallial cell. After middle May, mature microsporangium breaks down and pollens spread. In middle June, pollens on nucellus are budding. In late July, a body cell, a tube cell,a stalk cell and a prothallial cell are developed in thepollen tube on the top of nucellus. In middle August, the tip of pollen tube breaks. It releases materials consisting of sperm to archigonium chamber. The spermatozoids conjugate with ovum inside of archigonium to create zygote to complete impregnation. The mature female cone consists of several hundreds megasporophyte. The bottom of each megasporophyll comprises 2-6 ovules. Mature ovule has a sheet of integument. The space between integument is called micropyle. Integument covers nucellus. Space within nucellus is pollen-chamber. Female gametophyte locates at the centre of ovule. Micropylar end of female gametophyte has two to four archigoniums. Each archigonium contents of one ovum. Impregnation and development happen to them independently. Finally, only one mature embryo is formed. Female cone appears in middle February. Female cone initial comes from the top of plant. In early March, ovule initial appears under the epidermis of lateral basal site of megasporophyll. Ovule initial begins to protrusion in late March. Integument initial and nucellus initial are developed. The spaces between protruding integument build up micropyle spontaneously. In early April, centre of nucellus makes sporogenous cell. One of the sporogenous cell grows into a megaspore mother-cell in middle April. At that time, megaspore mother cell processes meiosis to form megaspore tetrad. Three of them degenerate to leave the one most nearest to chalaza end to form funtional megaspore. The funtional megaspore begin to go on karyokinesis to create free nucleus. In middle April, pollen chamber is build up in nucellus. In late May, the amount of free nucleus in female gametophyte is estimated to several thousands. Archigonium initial come appears on micropylar end of female gametophyll. Archigonium initial forms two primary neck cell and one central cell after abruption. In early June, central cell divides to a egg and a ventral-canal nucleus . Impregnation occurs in middle August. Proembryo starts to develop in late August. Germ matures in the coming February, including two cotyledons.