The answer is most likely "vascular system". Xylem tissue is a part of a plant's vascular system. Which is mostly responsiple for transportation (of water, food etc.) in a plant. The vascular system of plants is made of two tissues. One is Xylem and the other is Phloem. Phloem transports food from leaves to all other parts of the plant. And Xylem transports water and minerals (which are dissolved in the water) to the leaves.
So if you remove the Xylem tissue from a plant, its vascular system will be affected.