圧縮あて材仮道管の2次壁の形成 : らせん状のうねと裂目について

Abstract

圧縮あて材仮道管における特異な構造として知られている, いわゆる"らせん状裂目"の形成過程をスギ苗木を材料として各種の光学顕微鏡と透過型および走査型電子顕微鏡で観察した。観察に際しては, 試料の固定法やらせん状裂目の特異な構造にとくに留意した。そしてこのらせん状裂目はS_2層に生じた裂溝ではなく, S_2層形成初期からの細胞壁の不均一な肥厚に起因することを確かめた。観察結果の概略は以下のとおりである。(1) S_2層形成のごく初期に, 形成中の細胞壁内表面にすでに多数の, 巾の小さいらせん状のうね (ridge) と溝 (cavity) が観察された。またそのうねの先はとがっていた。(2) S_2層の肥厚が進むにつれて, このうねは櫛の歯状に高さがまし, それにはさまれた部分にらせん状裂目が形成されていた。うねの生長はその後しだいに不均一になり, 生長を続けるうねと, それらに押えられ生長を止めてしまううねとが見られる。生長を止めたうねの先端の所ではらせん状裂目があたかも分岐しているように見える。(3) S_2層形成の後期には, とくに巨大に生長するうねがあらわれ近くのうねをおおって発達する。(4) 細胞壁形成のすべての過程を通じて, 原形質膜は常に存在し, S_2層の不均一な肥厚の際には形成中の細胞壁内表面にそって著しい凹凸を示した。

The differentiation of compression wood tracheids, particularly the formation of their helical ridges and cavities, in Cryptomeria japonica was studied in detail by polarizing microscopy, scanning and transmission electron microscopy in close examination of fixation artefacts and of the misinterpretation resulting from the inclination of the ridges and cavities in the sections. The differentiating xylem was fixed by potassium permanganate or glutaraldehyde-osmium tetroxide, and embedded in epoxy resin. The inclined sections at an angle of 45 degrees to the fiber axis were used in order to observe the accurate cross view of the helical cavities. Results obtained are as follows: (1) Potassium permanganate fixation caused noticeable swelling of the cell wall lamellae (Fig. 4), whereas glutaraldehyde-osmium tetroxide fixation preserved the fine structure of both the wall and cytoplasm. (2) Although the fine structure of the ridges and cavities was obscured by their own inclination in the case of observations of the conventional cross- and radial-sections, this problem was solved by using so-called "inclined sections". (3) The cavities were clearly found at the very early stage of S2 layer deposition as the small interspaces among the small and edged helical ridges (Fig. 8, 9). This was also shown by observation of the differentiating radial file of the radial-longitudinal section using a polarizing microscope (Fig. 2). (4) The original ridges were developed toward inside like the teeth of a comb, leaving the cavities between them (Fig. 10). (5) In the final stage of S2 layer deposition, some of the ridges were extremely developed and the rest of them were suppressed by the developed ones (Fig. 11, 12). (6) Evidence of bursting open of fibrillar lamellae due to the tangential contraction was not observed at all during S2 layer deposition. (7) The plasma membrane existed in all stages of cell wall development, and they covered the ridge surfaces and penetrated deeply into the cavities (Fig. 12). (8) It is assumed from these observations that cavities do not originate from a tangential contraction of cell wall lamellae but are caused by an irregular deposition of the cell wall substance.