Tibetan rtswa ‘grass’

In this paper, Guillaume Jacques proposes that the Old Tibetan semi-vowel –^w– as part of a word onset is secondary, and that it has its origin in words ending in -u followed by -ba: he supposes the evolution Cu-ba > Cuwa > C^wa, for instance z^wa ‘nettle’ < zu-ba, r^wa ‘horn’ < ru-ba, gr^wa ‘corner’ < gru-ba. Another example is ‘grass’, OT rts^wa, which must then come from an earlier rtsu-ba. Tibetan rts^wa is compared to Chinese 草 *[tsʰ]ˤuʔ > tshawX > cǎo ‘grass, plants’ by Matisoff (here, p. 177) under a reconstruction PST *r-tswa-n, as part of a list of mostly spurious comparisons. In the case of ‘grass’, Matisoff got lucky, but only Jacques’s proposal makes sense of the phonology of this comparison, since OT -a does not otherwise correspond to OC -u.