transpiration and translocation

• pholem tubes transport foods
• (p) made of columns of elongated living cells with small pores in the end walls to allow cell sap to flow through
• (p) they transport food substances made in the leaves to the rest of the plant for immediate use or for storage
• (p) the transport goes in both directions
• (p) this process is called translocation
• (x) made of dead cells joined end to end with no end walls between them and a hole down the middle. they're strengthened with a material called lignin
• (x) they carry water and mineral ions from the roots to the stem and leaves
• (x) the movement of water from the roots, through the xylem and out of the leaves is called the transpiration stream
• 1. transpiration is caused by the evaporation and diffusion of water from a plant's surface. most transpiration happens at the leaves
• 2. this evaporation creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace it
• 3. this means that more water is drawn up from the roots and so there's a constant transpiration steam of water through the plant
• transpiration is just a side effect of the way leaves are adapted for photosynthesis. they have to have stomata in them so that gases can be exchanged more easily. because there's more water inside the plant than in the air outside, the water escapes from the leaves through the stomata by diffusion