SIEVE TUBES IN FOLIAR VEIN ENDINGS: REVIEW AND QUANTITATIVE SURVEY OF RUDBECKIA LACINIATA (ASTERACEAE)

Sieve tube distribution in foliar vein endings of angiosperms is mentioned in 33 publications. Their collective judgment is that sieve tubes may be lacking, may end partway along, or may extend to the vein tip. Most reports conclude that all vein endings are the same within a species, but a few studies mention variation. Only three studies include quantitative data, all based on reconstruction from microtome sections. The present quantitative study, the most extensive one to date, surveyed sieve tubes in vein endings from cleared and stained leaf pieces of Rudbeckia laciniata (Asteraceae, tribe Heliantheae), a North American perennial herb. Of 203 areoles sampled, 32 (15.8%) lacked vein endings whereas 171 (84.2%) had one to six endings per areole. There were 385 individual vein endings in the 171 areoles: 115 lacked sieve tubes, 38 had them to the vein tip, but in most (232) they ended at some intermediate point. Serial cross sections of the latter showed phloem parenchyma continuing beyond the sieve tubes for some distance, beyond which occurred a distal zone consisting solely of tracheary elements. R. laciniata vein endings exhibit the same range of variation reported for dicots in general. More studies combining clearings with selected sections are needed to establish the range of vein ending patterns among angiosperms.