CBSE Class 11-science Answered

Usually, the source is the part of a plant where food is synthesised and the sink is the part of the plant where the food is required or stored.

However, it can be reversed, especially during seasonal changes or according to the plant’s need.

In the early spring, sugar stored in the roots is sometimes used and acts as a source, and the buds of trees which need energy for growth and development of the photosynthetic apparatus act as a sink.

Hence, the source-sink relationship is variable in plants, and the direction of movement of food in the phloem is bidirectional.