Singapore's flora shares some affinity with the flora of southern Johore and the northern flora of Borneo. And indeed as those regional floras were better studied, only one species from that 1900 list of 33 proved to be endemic.
However, later on, Ridley himself, Holttum and few others revived the list of endemics with some of the new species that they described from Singapore. Up to recently, the "revived" list of endemics was 19 species.
Recently, Kiew & Turner (2003) scrutinised the list and report that only seven eventually proved to be endemic as knowledge of regional floras improved. The rest had their distributions widened to Peninsular Malaysia.
Sadly, of this seven, only one, an aquatic aroid recently described from Bukit Timah in 2001, is extant - Cryptocoryne xtimahensis Bastmeijer.
Even then, its long term status as an endemic is tenuous as the putative parents are also found in Southern Peninsular Malaysia; Cryptocoryne are known to hybridise readily. On top of that, it is also a very vulnerable species, growing in two adjacent small pools along just one stream.
Source: Ruth Kiew & Ian Mark Turner, 2003. Are any plants endemic to Singapore? The Gardens' Bulletin, 55(2): 173-184.
The seven endemic plants of Singapore
- extinct - Bolbitis xsingaporeansis (fern)
- extinct - Flickingeria laciniosa (epiphyte)
- extinct - Spatholobus ridleyi (climber)
- extinct - Strychnos ridleyi (climber)
- extinct - Thunbergia dasychlamys (climber)
- extinct - Tectaria griffithii var. singaporeana (fern)
- endangered - Cryptocoryne xtimahensis (aquatic)