纹孔

植物细胞的细胞壁上，未经次生加厚而留下的凹陷.初生细胞壁随细胞的生长而扩大，由原生质体产生的纤维素不断添加到初生壁上，但细胞壁的加厚并不是均匀的.很多地方留下没有增厚的空隙，这就是纹孔.纹孔通常呈小窝、缝隙或细管状.相邻的细胞壁其纹孔常成对地相互衔接，称纹孔对.纹孔可分为三种类型：（１）单纹孔，胞壁未加厚部分呈圆孔形或扁圆型，纹孔对中间由初生壁和中胶层所形成的纹孔膜隔开.（２）具缘纹孔，纹孔边缘的次生壁，向细胞腔内隆起呈拱架状，形成一个半圆的纹孔腔，纹孔腔有一纹孔口，纹孔膜中央加厚成为纹孔塞，因此从正面看呈三个同心圆状.纹孔塞仅在松柏类植物的管胞中才具有，其他裸子植物与被子植物则没有纹孔塞，故从正面仅看到两个同心圆.（３）半缘纹孔，在管胞或导管与薄壁细胞之间形成的纹孔.一边具缘，一边不具缘.一般情况下，纹孔膜、纹孔塞都处于纹孔对的中央位置.而在植物茎的心材上，大多数具缘纹孔对都偏向侧位，封住了纹孔口，特称封闭式纹孔，这种现象有利于防止水分散失，常与木材中心干枯有关.松柏类植物纹孔的数目和排列有很大变化，可分为梯状纹孔式、列对纹孔式和互利纹孔对式.The cell wall of plant cells without secondary thickening and the left depression. Primary cell wall with cell growth and the expansion of the cellulose produced by protoplast been added to the primary wall, but the cell wall thickening is not uniform . in many places there is no thickening of the gaps left behind, that is, pits. pit was usually nest, cracks or thin tubes. adjacent to the cell wall of their pits often link to each other in pairs, saying that pits right. pits can be divided into three types: (1) simple pits, cell wall was not thickened part of the hole-shaped or oblate type, pits right in the middle layer from the primary wall and the pit membrane formed separated. (2) a bordered pits, pits the edge of the secondary wall, the cell cavity was Gantry-like uplift, forming a semicircle of the pit chamber, pit cavity there is a pattern orifice, pit membrane thickening as the central pit plug, so from the front, was like three concentric circles. pit plug only in the tracheids of conifers only with other gymnosperms and angiosperms, there is no pit plug, so from the front only to see two concentric circles. (3 ) Semi-bordered pits in tracheids and parenchyma cells or duct formed between the pits. the side of a margin, while non-edge. In general, pit membrane, pit plugs are in the pits right of the central location. In the heartwood of the stems of plants, most bordered pit pairs are biased in favor lateral, sealed pattern openings, special called closed pit, this phenomenon helps prevent water loss, and often dry the timber center. Conifer pits on the number and arrangement of a great change, can be divided into scalariform pitting, columns and mutually beneficial to the pitting pitting right-style.