%0 Journal Article
%A Nina I. Gabarayeva
%A John R. Rowley and John J. Skvarla
%T Exine Development in Borago (Boraginaceae). 1. Microspore Tetrad Period
%D 1998
%J Taiwania
%V 43
%N 3
%P 203-214
%U https://taiwania.ntu.edu.tw/abstract/120
%X In the tetrad period of Borago officinalis L. microspores, procolumellae (exine tuft-units) 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.
%M doi:10.6165/tai.1998.43(3).203