Why does this happen?
1. Leaves of plants fall off in a particular season.
2. Fruits, flowers fall off after a certain period of time.
3. Substances like resin, gum, etc. are given out of the plant body.
Excretion is a simpler process in plants than in animals.
There is no special organ or system for excrtion in plants.
Gaseous substances are given out by diffusion.
Most of the waste substances of plants are stored in vacuoles of leaf-cells and in flowers, fruits and the bark of the stem. After some time these parts fall off.
Some other waste materials are stored in old and worn xylem in the form of resin and gum.
Some waste materials of plants are useful to humans,
e.g. gum, resin, latex of rubber, etc.