ACT Reading — one long informational passage, nine questions: why leaves turn

Sample question

Read the following article. Its paragraphs are numbered. [1] Every October, visitors crowd the back roads of Vermont and New Hampshire to see the hillsides turn. Many of them could tell you why it happens: the first frosts of fall, they would say, paint the leaves. It is a pleasing idea, and it is mostly wrong. Frost has something to do with the show, but chiefly as a spoiler. The colors are made, or uncovered, by the tree itself, as part of its preparations for winter. [2] All summer a leaf is a factory, and its most important machinery is chlorophyll, the green pigment that captures the energy of sunlight and uses it to turn water and carbon dioxide into sugar. Chlorophyll is not a durable substance. Sunlight breaks it down constantly, and the leaf constantly builds more, so that through June and July the supply stays high and the leaf stays green. [3] As summer ends, the nights lengthen and the air cools. For many trees the lengthening night is the decisive signal. The tree slows and then stops making new chlorophyll, and as the old supply breaks down, the green drains out of the leaf. What appears in its place was there all along. Leaves also contain carotenoids, yellow and orange pigments that help gather light during the summer but are hidden by the far larger amount of chlorophyll. When the green fades, the yellows of birches and hickories and the golds of many maples are simply revealed, the way a painted wall shows through when wallpaper is stripped away. [4] The reds and purples are a different story. They come from pigments called anthocyanins, and in most trees these are not present during the summer at all. They are manufactured in the fall. At about the time chlorophyll production stops, a layer of special cells begins to form where the leaf stalk joins the twig. This layer will eventually cut the leaf loose, but first it gradually blocks the veins that carry sugar out of the leaf. Sugar trapped in the leaf, together with bright light, supplies what many species, such as red maples, sumacs and sweetgums, need to build anthocyanins. [5] This is why the weather of a particular autumn matters so much. A run of warm, sunny days followed by cool nights that never quite reach freezing produces the most brilliant reds: the sun drives sugar production during the day, and the cool nights keep the sugar in the leaf. An unusually warm fall tends to dull the display, and an early hard frost can kill the leaves before much red has formed. Frost, in other words, is not the painter; at the wrong moment it ends the painting. [6] The reds raise a question the yellows do not. If a leaf is about to fall, why should a tree spend energy making a new pigment for it? One explanation holds that anthocyanins act as a sunscreen. Before a leaf drops, the tree draws valuable nutrients such as nitrogen out of it and stores them for spring, and this salvage work may go better if the leaf's weakened machinery is shaded from harsh light. Another explanation proposes that bright red is a signal to insects, such as aphids, that lay their eggs on trees in autumn, advertising that a tree is vigorous enough to defend itself. Many plant physiologists find the first idea more persuasive, since in some studies leaves unable to make the red pigment recovered less of their nitrogen, but the question is not settled. [7] So the visitors on the back roads are watching two processes at once: an unveiling of colors that were present all summer and a late burst of new color made at considerable cost. Neither has anything to do with frost, except that frost can bring both to an early end. The main purpose of the article is to

• persuade readers to visit Vermont and New Hampshire in October
• explain how the colors of autumn leaves are produced and correct a common belief about their cause
• argue that aphids are responsible for the red color of autumn leaves
• compare the autumn colors of New England with those of other regions