1 00:00:01,800 --> 00:00:07,480 Speaker 1: Welcome to brainstuft, a production of iHeart Radio, Hey, brain Stuff, 2 00:00:07,480 --> 00:00:12,080 Speaker 1: Flour and vog obam here enthusiastic snowball fights in winter 3 00:00:12,440 --> 00:00:16,000 Speaker 1: or blazy summer days at the beach. Notwithstanding, autumn might 4 00:00:16,040 --> 00:00:19,439 Speaker 1: be the most beloved season of the mall. Every year, 5 00:00:19,560 --> 00:00:22,600 Speaker 1: people in temperate climates watch in awe as the trees 6 00:00:22,680 --> 00:00:27,320 Speaker 1: begin their magical transformation from green powerhouses to veritable Morgese 7 00:00:27,360 --> 00:00:32,000 Speaker 1: boards of brilliant yellows, fiery oranges, and vibrant reds. It's 8 00:00:32,040 --> 00:00:35,479 Speaker 1: even a tourist attraction. The northeastern United States, for example, 9 00:00:35,600 --> 00:00:38,400 Speaker 1: hosts millions of leaf peepers from all over the world 10 00:00:38,560 --> 00:00:43,440 Speaker 1: who come to observe nature's fine art gallery. Myths about 11 00:00:43,440 --> 00:00:46,599 Speaker 1: why this occurs stretch back through time, from the hot 12 00:00:46,720 --> 00:00:49,640 Speaker 1: and O'shannee to the Algonquin to the Wye Dot. A 13 00:00:49,680 --> 00:00:53,440 Speaker 1: few Native American people's legends feature a wayward bear, sometimes 14 00:00:53,479 --> 00:00:56,400 Speaker 1: represented by ursa Major or the Big Dipper, which returns 15 00:00:56,440 --> 00:00:59,400 Speaker 1: to the sky every autumn. The legends say that the 16 00:00:59,440 --> 00:01:01,880 Speaker 1: bear was injured in a hunt or a fight, and 17 00:01:01,920 --> 00:01:07,000 Speaker 1: that's blood tends the leaves red each year. Much more recently, 18 00:01:07,120 --> 00:01:11,039 Speaker 1: researchers have sussed out the science behind these transformations, involving 19 00:01:11,040 --> 00:01:15,280 Speaker 1: photosynthesis and pigments. For a large part of the year, 20 00:01:15,520 --> 00:01:19,959 Speaker 1: leaves are a tree's workhorses, constantly converting carbon dioxide, water, 21 00:01:20,080 --> 00:01:24,080 Speaker 1: and sunlight into energy in the process called photosynthesis. The 22 00:01:24,200 --> 00:01:27,039 Speaker 1: key to this process is the pigment chlorophyll, which is 23 00:01:27,040 --> 00:01:29,600 Speaker 1: what gives leaves their green color for much of the year. 24 00:01:30,440 --> 00:01:33,120 Speaker 1: But while chlorophyll is the star of the show, it 25 00:01:33,240 --> 00:01:35,520 Speaker 1: has some help in the form of other pigments like 26 00:01:35,640 --> 00:01:41,280 Speaker 1: caroteene and xanthophill. Xanto is Greek for yellow, and Caroteene 27 00:01:41,319 --> 00:01:43,920 Speaker 1: is named for carrots because it's what gives them and 28 00:01:44,000 --> 00:01:48,000 Speaker 1: other things their orange color. These two pigments are always 29 00:01:48,040 --> 00:01:51,200 Speaker 1: present in leaves and help absorb sunlight, which they transfer 30 00:01:51,320 --> 00:01:56,640 Speaker 1: to chlorophyll for photosynthesis to occur. As summer nears its 31 00:01:56,840 --> 00:02:00,000 Speaker 1: end and days get shorter, the decreased amount of sunlight 32 00:02:00,160 --> 00:02:04,000 Speaker 1: incites trees to prepare for a sort of hibernation. The 33 00:02:04,080 --> 00:02:07,480 Speaker 1: leaves won't be able to continue photosynthesizing during winter due 34 00:02:07,480 --> 00:02:10,000 Speaker 1: to the dry air and lack of light, so the 35 00:02:10,040 --> 00:02:14,320 Speaker 1: tree does two things. First, it forms a separation layer 36 00:02:14,480 --> 00:02:17,040 Speaker 1: made of cork like cells at the base of each leaf, 37 00:02:17,080 --> 00:02:19,960 Speaker 1: to seal it off from the tree the second it 38 00:02:20,040 --> 00:02:23,120 Speaker 1: stops producing chlorophyll, since it won't need this pigment until 39 00:02:23,160 --> 00:02:25,320 Speaker 1: the days start to Lengthen once again in the spring, 40 00:02:26,360 --> 00:02:29,160 Speaker 1: with chlorophyll out of the picture, the yellow and orange 41 00:02:29,200 --> 00:02:34,120 Speaker 1: pigments get a chance to shine. The red hues which 42 00:02:34,160 --> 00:02:38,920 Speaker 1: come from pigments called anthocyanins, are slightly more complicated. Whereas 43 00:02:39,000 --> 00:02:43,000 Speaker 1: all trees contain chlorophyll, carotene, and xanthophill, and not all 44 00:02:43,000 --> 00:02:46,240 Speaker 1: of them produce anthocyanins, even the ones that do have 45 00:02:46,280 --> 00:02:51,000 Speaker 1: anti synons only produce it under certain conditions. Remember that 46 00:02:51,120 --> 00:02:53,320 Speaker 1: layer of cork like cells at the base of the leaf. 47 00:02:53,840 --> 00:02:56,480 Speaker 1: Its purpose is to protect the tree during the colder winter, 48 00:02:57,000 --> 00:03:00,600 Speaker 1: conserving energy and preventing the tree from drying out. And 49 00:03:00,639 --> 00:03:03,400 Speaker 1: when the separation layer is complete, the leaves fall off 50 00:03:03,400 --> 00:03:06,160 Speaker 1: of the tree. But before the leaves fall off and 51 00:03:06,200 --> 00:03:08,800 Speaker 1: the tree closes up shop, it wants to pull in 52 00:03:08,840 --> 00:03:11,760 Speaker 1: as much sugar and nutrients as possible from its leaves, 53 00:03:12,080 --> 00:03:16,440 Speaker 1: which is where the antisyonin comes into play. Although scientists 54 00:03:16,480 --> 00:03:20,680 Speaker 1: offer several different possible reasons for why some trees produce anthocyanins, 55 00:03:21,080 --> 00:03:24,120 Speaker 1: the prevailing theory is that these pigments protect the leaves 56 00:03:24,120 --> 00:03:27,079 Speaker 1: from excess sunlight and enable the trees to recover any 57 00:03:27,200 --> 00:03:30,919 Speaker 1: last remaining nutrients. The reason you'll see more vibrant reds 58 00:03:31,000 --> 00:03:34,360 Speaker 1: during some years is that stressors like an overabundance of 59 00:03:34,400 --> 00:03:38,560 Speaker 1: sunlight and dry weather increase the sugar concentration in tree sap, 60 00:03:38,880 --> 00:03:42,080 Speaker 1: triggering the tree to release more anthocyanins in a lastic 61 00:03:42,160 --> 00:03:44,600 Speaker 1: effort to gather up enough energy to get through the winter. 62 00:03:45,720 --> 00:03:49,280 Speaker 1: In addition, near freezing weather, low nutrient levels, and other 63 00:03:49,320 --> 00:03:54,360 Speaker 1: plant stressors seem to trigger increased levels of antisynins. So 64 00:03:54,520 --> 00:03:57,880 Speaker 1: if it's been especially rainy and overcast, you won't see 65 00:03:57,960 --> 00:04:01,480 Speaker 1: much red foliage. Without bright sunlight, the trees don't need 66 00:04:01,520 --> 00:04:04,560 Speaker 1: the added protection that the red pigments provide, so they 67 00:04:04,600 --> 00:04:08,600 Speaker 1: don't bother producing them. If autumn just isn't the same 68 00:04:08,640 --> 00:04:11,160 Speaker 1: for you without that splash of red, hopefully the weather 69 00:04:11,280 --> 00:04:19,320 Speaker 1: will collaborate. Today's episode is based on the article why 70 00:04:19,360 --> 00:04:21,400 Speaker 1: do Leaves change color in the Fall? On how stuff 71 00:04:21,400 --> 00:04:24,160 Speaker 1: works dot Com written by Jennifer Horton. Brain Stuff is 72 00:04:24,200 --> 00:04:26,240 Speaker 1: production of I Heart Radio and partnership with how stuff 73 00:04:26,240 --> 00:04:28,720 Speaker 1: works dot Com, and it's produced by Tyler Klang. For 74 00:04:28,920 --> 00:04:31,600 Speaker 1: more podcasts from My heart Radio, visit the heart Radio app, 75 00:04:31,680 --> 00:04:34,360 Speaker 1: Apple Podcasts, or wherever you listen to your favorite shows.