Final answer:
In spring and summer, leaves act as sources, producing photosynthates necessary for plant growth, and these sugars are transported to sinks such as roots and young shoots through the phloem.
Step-by-step explanation:
During spring and summer, leaves function as sources in plants. As a source, leaves produce photosynthates, which are sugars formed through photosynthesis.
These sugars are essential for the growth of the plant and are transported to different parts of the plant that are growing or storing energy, known as sinks, via the phloem in a process referred to as translocation. Sinks can include roots, young shoots, and developing seeds.
In spring and summer, when plants are undergoing a lot of growth, the leaves are usually the main photosynthetic organ, actively producing sugars.
These sugars are then translocated to other parts of the plant, such as the new leaves, shoot tips, and roots - wherever they are needed most for development and growth. Therefore, in these seasons, leaves serve as a crucial source.