Final answer:
The fleshy tissue parts of a plant primarily perform the function of storage.
Step-by-step explanation:
The function that is mostly performed in the fleshy tissue parts of a plant, rather than in the tissues that make up the upper layer of leaves, is storage. Fleshy tissues, such as the roots, stems, and some specialized structures like tubers or bulbs, store nutrients, water, and energy in the form of starch or glucose. These storage organs help plants survive during periods of adverse conditions, such as drought or winter.
Learn more about Plant physiology