%0 Journal Article
%A John R. Rowley
%A John J. Skvarla and Nina I. Gabarayeva
%T Exine Development in Borago (Boraginaceae)2. Free Microspore Stages.
%D 1999
%J Taiwania
%V 44
%N 2
%P 212-229
%U https://taiwania.ntu.edu.tw/abstract/134
%X  At the beginning of the free microspore period, when the callosic special cell wall is digested, darkly contrasted particles appear distally on each of the columellae. They are 15 to 20 nm in diameter and are connected to each other like beads on a string.  Several of such 'strings' form a bouquet-like formation on the distal end of columellae. We consider these particles to be involved with sporopollenin accumulation and refer to them as sporopollenin acceptor particles (SAPs).  SAPs appear to be associated with an increase in height of columellae and formation of gemmae.  By the end of exine development columellar height has increased 2 to 3 times.  In mature pollen gemmae have rounded ridges on their surface that are the same size as SAPs in the late microspore stage.
%M doi:10.6165/tai.1999.44(2).212