Actually, this would made a good fable, an anthropomorphic dialogue between fruit and leaves—“Who is greater?” A personified conversation, if you will, to determine which is the most important. Of course that's a rhetorical question. For a fruit bearing tree, both are essential.
The Scriptures often use the comparison of a righteous person and a tree. Like the palm, like the cedar in Lebanon, etc. with the unique characteristics of each. Fruit bearing has many analogies in Scripture. But leaves? Not so much. Psalm 1:3, however, launches our understanding of their importance. “He [the blessed man who doesn't walk in the counsel of the wicked] shall be like a tree planted by the rivers of water, that brings forth its fruit in its season, whose leaf also shall not wither; and whatever he does shall prosper.”
Psalm 92:12-15 echoes and embellishes the thought, describing the tree [righteous man] as flourishing, fruit bearing even in old age, and “full of sap and very green.” The latter phrase must apply to leaves. So why are leaves important to the entire healthy tree process? For the amazing process of Photosynthesis, which happens in the leaves of plants and trees whereby sunlight, water, and carbon dioxide from the air are converted into food and oxygen. Chlorophyll is a chemical that is in leaves throughout the year and that helps them make food through photosynthesis. It's also what makes leaves green. Leaves truly occupy an exalted position without which the tree wouldn't survive or flourish.
The Psalmist rightly describes a spiritually healthy person as “full of sap” which keeps the leaves looking green. If leaves wither, they obviously lack sap—as also might we, and then wilt and shrivel up, Our deficiency becomes apparent to all who see us. There is no hiding of the fact that we haven't drawn upon the “Rivers of Living Water” that Jesus said were essential, or spent time with our leaves lifted up to the Son-light. Or we distanced ourselves from that Source of water by not staying “firmly planted" there. A Christian with withering leaves is a poor or negative witness.
The advanced season of life is no excuse for not declaring the Lord's faithfulness and His solid Rock-ness. (Psalm 92:15) There is no expiration date on speaking for the Lord or living righteously for Him. In fact, in the autumn of our lives when leaves change into their most awesome, brilliant colors, is when the Christ-life within
us should come forth in its blazing fullness to attract others to Him. Some leaves, in contrast, turn brittle, dry, and brown, and fall to the ground without fulfilling their magnificent destiny.
Leaves change color because of the lack of chlorophyll, which is what makes the leaves green in the spring and summer months. The changing color of leaves depends on the presence of carotenoids which turn leaves brown, orange, and yellow or anthocyanins which turn them purple or red. Deciduous trees lose their leaves in autumn because as temperatures fall, the trees create a seal between the branch and its leaves that protects the branches from the freezing temperatures of winter.
There is a time and season for all things in nature and in natural created life into whom God breathed the breath of His life. Trees go with the flow, literally, with the flow of chlorophyll. In human life we have the joy and privilege to go with His flow as well by not only by continuing to bear good fruit in our advanced calendar years, but also in not allowing our leaves to wither. Best of all, to accept His grande finale plan to burst forth with dazzling, splendid, sensational colors during the summit season of our lives to attract others into God's Eternal Kingdom!