植物莖部第一道周皮之起源特別著重水筆仔周皮之發育過程

Abstract

中文摘要： 在本研究中曾就四十七種植物莖部第一道周皮的起源作一般性的探討，其中第一道周皮起源於次表皮(subepidermis)的最多，共有二十三種，包括喬木、灌木、藤本、草本、木質草本等各種生長習性(habit)。 圓葉?屎樹(Larianthus plagiophyllus)、呂宋莢?(Viburnum luzonicum)、毛女真(Ligustrum japonicum var. pubesecens)及楊桃 (Averrhoa carambola)的毛茸下方的部位比一般不具毛茸者先開始形成周皮，但是葡萄柚(Citrus paradisii)、水芫花(Pemphis acidula)、糯米糰(Gonostegia hirta)及九重葛(Bougainvillea spectabilis)的第一道周皮的起源則不受表皮上是否有毛茸影響。 周皮在整株植物體內的發育過程以水筆仔(Kandelia candel)做較詳細的觀察，選取一整株水筆仔並配合不同時期採集的另外幾株，以探討水筆仔第一道周皮發生的位置，及周皮發育的過程。從四株水筆仔嫩枝的橫切面歸納出水筆仔的第一道周皮有從表皮(epidermis)生成的，也有由次表皮生成的，不論從那一層細胞起源，第一道周皮在初期都只是局部性的生成，而並不形成完整的一整圈。 水筆仔第一道周皮的發生很早，在第一年生長季內已可在第一個節間(first internode)找到好幾處已平周分裂(periclinally divided)過一次的表皮細胞或是次表皮細胞；第一次平周分裂是等分裂(equa1 division)，新生的兩個細胞大小根相近，朝外的是木栓細胞(cork cell; phellem)，朝內的是木栓形成層(cork cambium; phellogen) ，木栓形成層繼續行平周分裂，起初大多向外增生木栓層的細胞，稍後也向內產生栓皮層(phe1loderm)。 水筆仔莖部成長的表皮細胞具有很厚的角質膜(cuticle membrane)，表皮及其表面的角質膜都可歷久而不剝落淨盡，在離莖頂約五公尺，樹齡已有七年，直徑約五公分的莖及別株寬約七公分的莖內，周皮雖已十分發達，仍能見到片段的表皮和覆在其外的角質膜。 水筆仔的繼起周皮(subsequent periderm)常緊靠著前一道周皮的栓皮層內側而生，繼起周皮的細胞形態與已有的周皮的細胞很相似，基於這兩原因，各道繼起周皮之間的界限十分不明顯。水筆仔的栓皮層細胞起初很少，後來則偶有增生得比木栓層快，而在數量上超越木栓層的現象，但這也只是局部性的，並非整圈都如此，並且，這現象不一定只在老莖內出現，只有一圈半生長輪的部分嫩莖內也會有這現象。 水筆仔的木栓層及栓皮層都有成群的石細胞(brachysclereids; stone cell)散生在組織間。水筆仔全株的組織都含有單寧質(tannins)，而樹皮內含的量尤其多，不但栓皮細胞含有單寧，木栓細胞也含有，而且因單寧含量及貯存場所的不同，可將木栓薄壁細胞區分為兩類：一類存單寧於細胞壁，先貯於切線細胞壁，繼而徑線細胞壁也開始貯存；另一類則存單寧於細胞質或液胞內，貯於細胞質內的呈顆粒狀，貯於液胞內的在切片上看去整個細胞似乎塞滿了單寧。離木栓形成層較遠之栓皮層的細胞排列及形狀極為不規則，在較老的莖內變化尤其劇烈，常常形成許多空隙，而成通氣組織(aerenchyma)，但在更老的莖內，栓皮細胞及皮層細胞(二者有時不易區分)又有區域性的連續行垂周分裂(anticlinal division)以致新細胞填塞住細胞間隙的現象。有時，栓皮層與皮層、初生韌皮部之間很難區分。 木栓層在最初的七年內幾乎沒有剝落過，但在更老的莖，特別是靠近土表的部分，木栓層仍會從外開始一小塊一小塊的剝落，整個樹皮的外觀便由光滑轉為十分粗糙。

Abstract The origin of the first periderm has been widely investigated, but mainly on dicotyledonous stems. The first periderm in 23 out of the 47 plants investigated were initiated in the subepidermis. These included all, types of habit such as trees, shrubs, herbs, herbaceous and woody vines. In. Viburnum luzonicum, Larianthus plagiophyllus, Averrhoa carambola, and Ligustrum japonicum var. pubescens the regions located immediately beneath the trichomes formed periderm earlier than in other places, whereas its site of initiation in Pemphis acidula, Citrus paradisii, Gonostegia hirta and Bougainvillea spectabilis was not affected by the presence of trichomes. The formation of the first periderm as well as the later developmental pattern of subsequent periderms in Kandelia candel was not only studied on an entire plant but also in association with some twigs obtained at various seasons. Examining the transections of young stems of four Kandelia plants, it was observed that the first periderm originated either in the epidermis or subepidermal layer. Both the epidermal and subepidermal origins are found to be located in the same section in some twigs. The indication of the first periderm in Kandelia is as early as during its first growing season. A two-layered periderm can definitely be observed in its first internode.. Its first periclinal division is an equal division. The outer cell becomes the phellem and the inner one functions as the phellogen which continues to produce phellem cells outwards and phelloderm cells inwards. The well-developed epidermis is covered with a thick cuticle. This remains on the trunk surf ace for a long period, since the broken, discontinuous epidermis can always be recognized in. stems of about 5 cm in diameter, which are 7 years old. The subsequent phellogen of Kandelia starts to divide, with the inner cell layer located very close to the former periderm. The new periderm cells resemble the former periderm in cell lineage and appearance. So that it is difficult to mark out one periderm from its adjacent subsequent periderms. The cell layer in an. early formed phelloderm is always scanty. But in the deeper subsequent periderms, the phellogen forms a large number of phelloderm cells in some regions and produces only a few cell layers in the other places as seen at the same level of a section. The brachysclereids occur in groups and intermingled with parenchymatous phellem as well as the phelloderm in Kandelia, The brachysclereids containing periderm is found in both young and old stems. The tannin-containing cells are seen in all parts of the plant. They occur in extensive continuous masses in the phelloderm. Cork cells also contain tannins. Tannin parenchyma in the phellem may be divided into two types: one, having tannin on the walls; and the other having tannin in. either the cytoplasm or vacuole. The phelloderm cells become irregular in outline and turn into an aerenchymatous layer in later stages of development. In an older stem, it is difficult to indicate the boundary between phelloderm and its adjacent cortex. Some cells in both of these tissues divide anticlinally and fill in. the intercellular spaces. Finally the boundary lines between the phellodem, cortex, and primary and secondary phloem become less clear because of these developmental processes. It appears that the first periderm remains on the trunk as long as seven years though the additional subsequent periderms are formed continuously. The sloughing off process of the outer layers of the periderm is seen on older trunk surfaces. The outer layers separate from the inner portions of the bark in. the form of relatively large sheets, consequently the surface of the trunk becomes rough and scaly.