The part of a green plant that shows the greatest increase in chloroplasts by the end of spring is the leaves.

Here's why:

* Chloroplasts are the sites of photosynthesis: They contain chlorophyll, the pigment that captures sunlight to produce energy for the plant.

* Leaves are the primary photosynthetic organs: They are broad and flat, maximizing their surface area for light absorption.

* Spring is the season of growth: As days get longer and warmer, plants increase their rate of photosynthesis to support rapid growth. To facilitate this, they increase the number of chloroplasts in their leaves.

So, while other parts of the plant may have some chloroplasts, the leaves experience the most significant increase in chloroplast numbers during spring.