How will you measure the rate of growth? Describe an instrument used to measure the increase in height of an angiospermic plant.

Growth is a quantitative phenomenon can be measured in relation to time. It can be measured in terms of
1) Increase in length or height 
2) Increase in area or volume 
3) Increase in the number of cells
The length of a plant or plant organ can be easily measured by an ordinary measuring scale.
For accurate measurement of an increase in height or length of an angiospermic plant, a specially designed equipment called auxanometer can be used. 
It can be used to measure the rate of growth of shoot of plants. 
A thread is tied to the growing tip of a potted plant and the thread is hung on the pulley of auxanometer. The other end of the thread is tied to a weight. The pulley is fixed with a long needle, which slides over a graduated arc. As the plant grows in length the weight pulls the thread down. The movement of the needle is read on the scale of the arc which indicates the growth.