鳳尾蕨科葉部假脈與矽堆積形式之研究

Abstract

「假脈」(false vein) 泛指植物葉片上貌似葉脈，卻不具有疏導功能之構造。從過去的研究中，大致可將假脈分為兩大類型 (1) 回脈 (recurrent vein)：由相鄰小羽片之邊緣互相癒合而成，例如觀音座蓮屬 (Angiopteris) 之假脈；(2) 假支脈 (false veinlet)：由支脈退化而成，例如膜蕨科 (Hymenophyllaceae) 之假脈。而鳳尾蕨科之假脈則不同於上述兩大類型，是由表皮異形細胞 (epidermal idioblast) 所構成。於廣義的鳳尾蕨科 (Pteridaceae) 中，假脈可見於鳳尾蕨屬 (Pteris)、鐵線蕨屬 (Adiantum) 以及書帶蕨類群 (vittarioids)；本論文利用能量色散X光光譜儀 (EDX) 元素分析，配合組織化學染色的結果，證實此三類群的假脈均是由具二氧化矽堆積的矽異形細胞 (spicular cell, SC) 所構成。本論文將此構成假脈之細胞稱為脈間矽異形細胞 (interveinal spicular cell, ISC)，而將分佈於葉脈上的矽異形細胞稱為脈上矽異形細胞 (veinal spicular cell, VSC)。 為進一步探討鳳尾蕨科假脈、脈間矽異形細胞與脈上矽異形細胞之間的關係及其於鳳尾蕨科內的演化，本論文選取19屬82種的鳳尾蕨科植物，觀察其葉表面之二氧化矽堆積型式、一般表皮細胞形態以及脈上表皮細胞形態；同時，也開發出利用偵測背向散射電子 (backscattered electron) 之桌上型掃描式電子顯微鏡 (tabletop SEM) 配合後冷卻技術 (post-cooling method) 之快速檢測法，該方法利用背向散射電子訊號強弱與平均原子數 (average atomic number, Z number) 之相關性得到矽異形細胞與一般表皮細胞具有明顯對比之影像。 實驗的結果發現，約有三分之二的鳳尾蕨科植物葉表具有固定型式的二氧化矽堆積；而依據二氧化矽的堆積型式可將鳳尾蕨科分成四大類：(I) 無固定型式的二氧化矽堆積；(II) 二氧化矽堆積於一般表皮細胞邊緣而無矽異形細胞；(III) 具有脈上矽異形細胞但不具有脈間矽異形細胞；(IV) 具有脈間矽異形細胞。其中具有矽異形細胞之類群 (第III、IV類型) 均集中在鳳尾蕨支 (Pterioids, PT clade) 以及鐵線蕨支 (Adiantoids, AD clade) 之中，包含了孔雀鳳尾蕨屬 (Actiniopteris)、金粉蕨屬 (Onychium)、粉葉蕨屬 (Pityrogramma)、鳳尾蕨屬、鐵線蕨屬以及書帶蕨類群。重建特徵演化樹的結果則顯示鳳尾蕨支與鐵線蕨支之矽異形細胞應為不同起源，其中鳳尾蕨支內的矽異形細胞亦可能為多次起源。 本實驗中亦發現鳳尾蕨科之矽異形細胞具有幾項特色：(1) 矽異形細胞之長軸多與葉片側脈方向平行 (2) 發現多數鐵線蕨屬以及鳳尾蕨屬植物僅下表皮具有脈上矽異形細胞；(3) 具有脈間矽異形細胞 (假脈) 的種類會同時具有脈上矽異形細胞；(4) 長條形的側脈上表皮細胞與矽異形細胞具有相關性，僅具脈上矽異形細胞的分類群 (第III類型) 之脈上表皮細胞均較一般表皮細胞細長；具有脈間矽異形細胞之物種之脈上表皮細胞亦為長條形但形態與一般表皮細胞常無明顯形態差異。綜合以上特徵推測，鳳尾蕨科傾向將二氧化矽堆積於葉下表皮與葉脈平行之表皮細胞上，並形成矽異形細胞，該特徵可能與加強葉片的支持力有關。

“False veins” are vein-like structures without vascular bundle and conductive function. According to previous studies, false veins can roughly divide into two major types: (1) recurrent vein: form by fusion of leaflets, for instance, false veins of Angiopteris; (2) false veinlet: form by reduced veinlets, for instance, false veins of Hymenophyllaceae. False vein of Pteridaceae is different from the two major types of false veins, which is composed by epidermal idioblasts. In Pteridaceae s. l., false veins can be found in Pteris, Adiantum, and vittarioids; in this thesis, Energy Dispersive X-ray (EDX) element analysis and histochemical studies were used to confirm the false veins of these three taxa are composed of spicular cells (SC) – a kind of long, silica depositing epidermal idioblast. We named the spicular cells which forming false veins interveinal spicular cells (ISC), and the spicular cells on veins are veinal spicular cells (VSC) in this thesis. In order to know the evolutionary relationships between false vein, ISC, and VSC in Pteridaceae, 19 grnera and 82 species in Pteridaceae were sampled in this study; silica deposition types, ordinary epidermal cell morphology and veinal epidermal cells morphology of these species were surveyed. On the other hand, a convenient method was developed for this goal. In this method, a tabletop SEM with backscattered electron detector accompanied by post-cooling method was used. Because the backscattered electron image (BEI) is contrast by the difference of average atom number (Z number), the morphology and distribution of SC can be easily detected in this method. In our result, about two of three species in Pteridaceae had specific silica deposition patterns in epidermis. Four silica deposition types were identified in this study: (I) no special silica deposition pattern; (II) silica deposit at cell margin of ordinary epidermal cells but no SC present; (III) have VSC but no ISC; (IV) have ISC. All of the taxa have SC are belong to PT clade or AD clade in Pteridaceae, including Actiniopteris, Onychium, Pityrogramma, Pteris, Adiantum, and vittarioids. The results of character evolution history reconstruction shows the SC in PT and AD clade might be a homoplasy character; also, the SC in PT clade might have multiple origins. SC in Pteridaceae has some special characteristics: (1) the cell long axes usually run parallel to the lateral veins; (2) Most of Pteris and Adianum species have VSC only in lower epidermis; (3) Species have ISC (false veins) also have VSC. (4) Presence of spicular cells is highly correlated with the long veinal epidermal cells. Species have type III silica deposition type usually have longer and narrower veinal epidermal cell compared with ordinary epidermal cells; Species have type IV silica deposition type also have long epidermal cells, but not significantly different from ordinary epidermal cells. Based on these characteristics observed, species in Pteridaceae tended to accumulate and deposit silica in long cells in lower epidermis and forming spicular cells. This might helps them to enhance the supporting function of their lateral veins.