Sallow as an English name for willows has been applied to several species. Off the top of my head I can think of common sallow for Salix cinerea ssp. atrocinerea, eared sallow for S. aurita and great sallow as an alternative name for the goat willow, S. caprea. The latter, to contradict our learned friend above, is not the weeping willow, that epithet belonging to the very different S. babylonica (or a hybrid) as has been stated before. From all that's been said in the thread it would appear that Yeats would have had little justification for inserting that 'e' if he'd intended a connection with willows. But did he? :-)