Mass flow hypothesis was proposed by E. Munch (1930) and was elaborated by Crafts (1938). According to this hypothesis, organic substances move from the region of high osmotic pressure to the region of low osmotic pressure in a mass flow due to the development of a gradient of turgor pressure. Sieve tube system is fully adapted to mass flow of solutes. Here, the vacuoles are fully permeable because of the absence of tonoplast (Esau, 1966). A continuous high osmotic concentration is present in the source or supply region, e.g., mesophyll cells (due to photosynthesis). The organic substances present in them are passed into the sieve tubes through their companion cells by an active process. A high osmotic concentration, therefore, develops in the sieve tubes of the source. The sieve tubes absorb water from the surrounding xylem and develop a high turgor pressure. It causes the flow of organic solution towards the area of low turgor pressure. A low trugor pressure is maintained in the sink region by converting soluble organic substances into insoluble form. Water passes back into xylem.