Abstrad: Tumors induced by the wild-type strain C58 of Agrobacterium tumefaciens in hypocotyls of Ricinus communisL.were investigated structurally and functionally with respect to xylem differentiation, cuticle and stomata development, water pathway and transpiration. Clearing of tissue with lactic acid and staining with lacmoid revealed a continuation of stem xylem into differentiated bundles in the tumor. Under the influence of tumors the host xylem below the tumors increased considerably in size. Transport of negatively-charged dyes, amido black, acid fuchsin and the fluorescent pyrenetrisulfonate demonstrated a continuous water flow through the vessels from the stem into the tumor, and up to its surface. Infrared thermography and quantitative measurements of transpiration revealed that transpiration was about 15 times and 7.5 times higher at the tumor surface in comparison to host leaves and to leaves of non-infected plants, respectively. LeafCO 2 assimilation rate remained unaffected by tumorisation. Tumor growth caused disruption of the epidermis, which did not regenerate and hence no cuticle developed to protect against water loss. Stomata located at the tumor rim hypertrophied and lost their function'. Tumors are thus characterised as being structurally and functionally strong pathological water sinks on their host plant.