Why do trees lose their leaves? In certain parts of our world, known as temperate zones, trees must have some sort of protection to survive the freezing temperatures and cold, drying winds of winter. Stems and buds have coatings and structures that allow them to withstand harsh conditions. Broad flat leaves are quite delicate and could freeze in the winter, so the trees either lose their leaves all at once, or they produce small, narrow, flattened leaves with extra protection to survive the winter. Leaf abscission refers to the loss of leaves in deciduous trees. It's preceded by events that protect sites where leaves were once attached. Where the petiole joins the stem, an abscission layer forms, consisting of weak, colorless, thin-walled cells that allow the leaf to separate easily when a gust of wind comes along. Meanwhile, a protective layer of densely colored cells has formed between the abscission layer and the stem. These protective cells are filled with waxy material that forms a thick seal over the place where the leaf will detach. New growth comes from the axillary bud that's already formed in the angle between the leaf and the stem. Think of losing a leaf as scraping your skin, only to find a protective "scab" already in place. Now let's watch leaf abscission. Notice that the leaf changes color during abscission. Why do leaves change color before they fall? During the autumn months, when temperatures begin to decrease and daylight diminishes, the abscission layer begins to form, slowly cutting off the transport of water and sugars between leaf and stem, and signaling the end of sugar production in the leaves. At the same time, chlorophyll breaks down and the intense green color of the leaf begins to fade. With the fading of the chlorophyll, we begin to see the yellow and orange hues of the carotenoids, pigments that were present all along in the chloroplasts. Simultaneously, if the abscission layer forms before all the chlorophyll has broken down and while the days are still sunny and warm, sugars may be trapped in the vacuole of each palisade cell. If sugars are trapped and prevented from moving away from the blade to the stem, their relative concentration within each cell increases and triggers the production of deep purple or scarlet red anthocyanin pigments. The combination of yellow, orange, red, and purple within dying leaves provides spectacular fall color in some parts of the world. Sometimes we see colorful patterns in a single leaf, as in this ornamental pear.
Table of contents
- 1. Introduction to Biology2h 40m
- 2. Chemistry3h 40m
- 3. Water1h 26m
- 4. Biomolecules2h 23m
- 5. Cell Components2h 26m
- 6. The Membrane2h 31m
- 7. Energy and Metabolism2h 0m
- 8. Respiration2h 40m
- 9. Photosynthesis2h 49m
- 10. Cell Signaling59m
- 11. Cell Division2h 47m
- 12. Meiosis2h 0m
- 13. Mendelian Genetics4h 41m
- Introduction to Mendel's Experiments7m
- Genotype vs. Phenotype17m
- Punnett Squares13m
- Mendel's Experiments26m
- Mendel's Laws18m
- Monohybrid Crosses16m
- Test Crosses14m
- Dihybrid Crosses20m
- Punnett Square Probability26m
- Incomplete Dominance vs. Codominance20m
- Epistasis7m
- Non-Mendelian Genetics12m
- Pedigrees6m
- Autosomal Inheritance21m
- Sex-Linked Inheritance43m
- X-Inactivation9m
- 14. DNA Synthesis2h 27m
- 15. Gene Expression3h 20m
- 16. Regulation of Expression3h 31m
- Introduction to Regulation of Gene Expression13m
- Prokaryotic Gene Regulation via Operons27m
- The Lac Operon21m
- Glucose's Impact on Lac Operon25m
- The Trp Operon20m
- Review of the Lac Operon & Trp Operon11m
- Introduction to Eukaryotic Gene Regulation9m
- Eukaryotic Chromatin Modifications16m
- Eukaryotic Transcriptional Control22m
- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
- 17. Viruses37m
- 18. Biotechnology2h 58m
- 19. Genomics17m
- 20. Development1h 5m
- 21. Evolution3h 1m
- 22. Evolution of Populations3h 52m
- 23. Speciation1h 37m
- 24. History of Life on Earth2h 6m
- 25. Phylogeny2h 31m
- 26. Prokaryotes4h 59m
- 27. Protists1h 12m
- 28. Plants1h 22m
- 29. Fungi36m
- 30. Overview of Animals34m
- 31. Invertebrates1h 2m
- 32. Vertebrates50m
- 33. Plant Anatomy1h 3m
- 34. Vascular Plant Transport2m
- 35. Soil37m
- 36. Plant Reproduction47m
- 37. Plant Sensation and Response1h 9m
- 38. Animal Form and Function1h 19m
- 39. Digestive System10m
- 40. Circulatory System1h 57m
- 41. Immune System1h 12m
- 42. Osmoregulation and Excretion50m
- 43. Endocrine System4m
- 44. Animal Reproduction2m
- 45. Nervous System55m
- 46. Sensory Systems46m
- 47. Muscle Systems23m
- 48. Ecology3h 11m
- Introduction to Ecology20m
- Biogeography14m
- Earth's Climate Patterns50m
- Introduction to Terrestrial Biomes10m
- Terrestrial Biomes: Near Equator13m
- Terrestrial Biomes: Temperate Regions10m
- Terrestrial Biomes: Northern Regions15m
- Introduction to Aquatic Biomes27m
- Freshwater Aquatic Biomes14m
- Marine Aquatic Biomes13m
- 49. Animal Behavior28m
- 50. Population Ecology3h 41m
- Introduction to Population Ecology28m
- Population Sampling Methods23m
- Life History12m
- Population Demography17m
- Factors Limiting Population Growth14m
- Introduction to Population Growth Models22m
- Linear Population Growth6m
- Exponential Population Growth29m
- Logistic Population Growth32m
- r/K Selection10m
- The Human Population22m
- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
- Introduction to Community Interactions9m
- Community Interactions: Competition (-/-)38m
- Community Interactions: Exploitation (+/-)23m
- Community Interactions: Mutualism (+/+) & Commensalism (+/0)9m
- Community Structure35m
- Community Dynamics26m
- Geographic Impact on Communities21m
- 52. Ecosystems2h 36m
- 53. Conservation Biology24m
33. Plant Anatomy
Roots and Shoots
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