Unanswered is 19d
18a Divine aroma's recurring feature in genus of herbs (8) 7th letter would give the first 'I'
22a Plant found in Australia, but not America or Thailand (6) 5th letter would give second 'I'
25a Vegetable offshoot started without germinating initially (6,4) 9th letter would give the 'A'
27a Thatcher would make use of this study in speech (4) 3rd letter would give the 'E'