The sap can be divided into parts. There is the sap that rises up from the roots to the shoot and the sap that flows from the leaves to all the other parts of the plant. The sap rising from the roots carries the water and the minerals that the roots have taken in. The sap leaving the leaves is rich in food that the leaves have made.

Sap travels in tubes inside the plant. One set takes the sap from the roots the other set take the sap from the leaves.

Greenfly are adapted to feeding on the sap from the leaves. A greenfly has a mouth like a hollow needle. It sticks its mouth through the plant surface and into the tubes carrying the sap to take a drink. The food in the sap is very dilute, so the green fly has to drink large amounts. This makes the greenfly also excrete large amounts and make a sweet sticky substance on the shoot. This substance is called honeydew but it has nothing to do with nectar or bees.