This paper presents a full version of the graviosmotic hypothesis postulated earlier in outline (Kargol 1978), which concerns xylem rise of water in plants. According to this hypothesis water is transported by xylem vessels (at respective development stages of these vessels) using the graviosmotic mechanisms. A detailed description of hypothesis is introduced by a discussion of the development stages of xylem vessels and a presentation of graviosmotic mechanisms postulated to be involved. These mechanisms include: convective graviosmosis and related effects, gravidiffusional graviosmosis, and osmotic transport aided by gravitational force in multi-membrane systems. The presented hypothesis does not contradict the theory of transpiration-cohesion or that of root pressure; rather, it is complementary to them.