玻璃苣屬（紫草科）之花粉外壁之發育. 1.小孢子四分孢子期

Exine Development in Borago (Boraginaceae).1. Microspore Tetrad Period

玻璃苣小孢子在四分孢子期時，原柱狀層直徑為70至100nm，具有條狀或螺旋狀的次級構造。網叢具有圓形的中心帶，初期對比度較強，而外環帶對比度較弱。當柱叢高度增加時，其頂端直徑增為150至200nm，而基部新形成的部分則仍為70至100nm，此時原柱狀層為圓錐狀。在形狀和高度改變時。柱叢的對比度亦產生逆轉變化，即中心帶對比度變弱而外環帶對比度則變強。外環帶呈條狀或螺旋狀。蓋頂層的形成包含了柱叢頂端的延長與升高，及其對比度強弱的轉變，因此外環帶呈較深的對比度。原柱狀層由原生質膜至胼胝質壁之厚度為0.4至0.5µm高。由斜切面觀察，原柱狀層頂端部分的構造交錯而呈多角型。當小孢子母細胞外的胼胝質壁逐漸消失時，蓋頂層變得更顯著，而基層和內層也開始明顯。花粉的萌芽孔在胼胝質壁消失之前變得較複雜，當其形成時，原生質膜和胼胝質壁僅相隔50nm，而在這些初形成的萌芽孔邊緣，臘梅醣基質較尖細，高約50nm，但是在孔間區域的原柱狀層則有150nm高。胼胝質壁一直持續到小孢子釋出才消失。

In the tetrad period of Borago officinalis L. microspores, procolumellae (exine tuftunits) are 70-100 nm in diameter and have striated or coiled substructures. Tufts have a circular core zone that initially can be strongly contrasted and an outer (binder) zone that contrasts weakly. During increase in tuft height the early formed distal diameter of tufts becomes 150 to 200 nm while the new basal part is 70 to 100 nm. Thus for a time these procolumellae are cone-like. Along with this change in shape and height there is a reversal of contrast. For a time the core is only weakly contrasted while the binder zone can be strongly contrasted. It is the binder zone that is striated or coiled. Tectal formation involves elongation or elevation of units distally and reversal of contrast. The binder zone can be darkly contrasted. The procolumellae are now 0.4 to 0.5 µm in height from plasma membrane to the callose special cell wall. The distal portion of procolumellae interdigitate producing a polygonal pattern as seen in oblique sections. A tectum becomes prominent and a foot layer and endexine begin to be evident during the period of loss of the microspore mother cell callose envelope. The apertures become complex before the end of the callose period. As they are formed the plasma membrane is separated from the callose by only about 50 nm. At the margin of these young apertures the glycocalyx matrix tapers from the about 50 nm to the height of the interapertural procolumellae (about 150 nm). The callose of the special cell wall remains until the free microspore period.