アオギリの木部分化過程における道管の発達

Abstract

層階状構造をもつアオギリの木部分化過程における道管の発達とその周囲の細胞の形, 配列の変化を調べた。春材道管の発達経過は次のとおりであった。盛んに母細胞分裂が行なわれている分裂木部中に半径方向に拡大(表面成長)を始めた細胞がグループ状にあらわれる。その中の1 - 2個の細胞がそれぞれ発達して道管となる。この際, 表面生長を始めた細胞を含む半径列の細胞数増加がおさえられるが, 一方それよりはなれた側方の半径列は盛んな分裂による多数の新生細胞をもつ。これを分化中の道管要素D. V. E. の発達第一段階とする。D. V. E. は次の段階で接線方向に大きくなる (第二段階)。この時, D. V. E. は隣接半径列の細胞間に侵入生長をして行くが, D. V. E. および側方の半径列に形成された多数の細胞の半径方向生長 (表面生長木部Iの段階) により形成層が外方に押しやられると, 細胞数の少ないD. V. E. 隣接半径列は分離され, D. V. E. にスペースを提供するものと考えられる。D. V. E. の周囲を埋めているmatrixの細胞は表面生長をほぼ終えた後, 二次壁を形成しないでD. V. E. の発達を待つ (表面生長木部 II)。この時D. V. E. はこれらの細胞を押し拡げ, 半径方向にも大きくなり形を整える (第三段階)。matrixの細胞は全体に半径方向に引張られた様子を示し, 細胞配列の調整が進行する。次にD. V. E. が二次壁の形成を始め (第四段階), それが次第に周囲の細胞に及んで行く。比較のために調べたハリエンジュの春材道管についても同様の発達パターンが認められた。しかし, ハリエンジュではD. V. E. 第三段階 (表面生長木部 II) において, matrix細胞のうち繊維細胞となるものは著しく伸長し, 木部の半径方向生長および細胞配列の調整に大きく影響する。

Development of the vessel element and cellular adjustment in xylem differentiation were studied in wood of Firmiana platanifolia SCHOTT et ENDL. which has a conspicuous storied structure. Thin sections about 2 μ thick were taken from the specimens embedded in Epon 812 by glass-knife cutting and observed in FUJITA'S iodine staining method. Development of the early wood vessel is diagrammatically represented in Fig. 4. First, several cells in the dividing xylem stop dividing and begin to enlarge radially. One of them continues to enlarge and finally differentiates into a vessel element. The preceding enlargment of the cells in the dividing xylem decreases cell production in those radial files to which they belong, while in other files on either side of them, mother-cell division continues ordinarily. In the next stage, the differentiating vessel element (D. V. E.) begins to enlarge tangentially. The outward displacement of the cambium due to radial enlargement of the cells produced in the ordinary files and the intrusive growth of D. V. E., separate the adjacent files in a decreased cell-productions. In the third stage, the D. V. E. expands radially, as the space is provided by progressive radial enlargmeent of new cells, one after another, in the ordinary files. The turgid D. V. E. at this stage vigorously expands, pushing back the matrix cells which have already finished lateral enlargement but remain in the primary wall state. It often pushes the cambial zone outwards. During this stage a new cellular arrangement is brought about. Finally, the secondary wall formation occurs, firstly in the D. V. E. and then spreading gradually over the matrix cells. Development of the vessel element in Robinia pseudo-acacia was also studied and confirmed to be similar to that in Firmiana platanifolia. In its differentiating xylem, however, a remarkable elongation of differentiating fibers also causes the outward displacement of the cambium and plays a great part in cellular adjustment.