Plants are described in many texts of Jainism, Buddhism (although Schmithausen has shown how they do not represent the final opinion) and "Hinduism" as ekendriya, that is as "endowed with a single faculty". Jainas are clear about the identification of this single faculty as touch (sparśa in Sanskrit) and many texts agree in attributing to plants the touch-related sensations of heat/cold, etc. In this sense, the fact of having just one sense faculty agrees with the collocation of plants on the bottom end of the evolutionary scale of living beings, together with other subtle entities without any individual body. However, it is difficult to see why touch should be the first sense faculty to appear in a living being. In fact, touch is usually connected with air (vāyu) and both are in the middle of the list of elements/sense faculties. I have already discussed elsewhere (in Italian) the order of the gross elements as connected with the subtle ones and the sensory faculties.
This issue, together with the consideration that Jainism usually preserves many features of the oldest strata of Indian philosophy, points at the possibility that ether (together with sound as its content and ear as the corresponding sensory faculty) has been added on top of the list of gross elements (ether/air/fire/water/earth). At an earlier time, air would have been the first of the list and, consequently, touch would have been the first sensory faculty to emerge in the simplest living beings.