Modern travellers bear witness to a gradual progress of desiccation in the Tibetan uplands.
The occurrence of characteristic algae at different levels constituting the zones to which reference has already been made, is probably in part an expression of the fact that different species vary in the capacity to resist desiccation from exposure.
Algae of more delicate texture than either Fucaceae or Laminariaceae also occur in the region exposed by the ebb of the tide, but these secure their exemption from desiccation either by retaining water in their meshes by capillary attraction, as in the case of Pilayella, or by growing among the tangles of the larger Fucaceae, as in the case of Polysiphonia fastigiate, or by growing in dense masses on rocks, as in the case of Laurencia pinnatifida.
During their removal it is important that the roots be covered, if only to prevent desiccation by the air.
This was, however, more probably due to the deterioration of the country through desiccation, which has forced the settled population farther westward, where Sana became the centre of the later Himyaritic kingdom.