LE 36-1420 µmXerophyte Adaptations ThatReduce Transpiration• Xerophytes are plants adapted to aridclimates• They have leaf modifications thatreduce the rate of transpiration• Their stomata are concentrated on thelower leaf surface, often in depressionsthat provide shelter from dry windLE 36-16CuticleUpper epidermal tissueLower epidermaltissueTrichomes(“hairs”)Stomata100 µmConcept 36.5: Organic nutrientsare translocated through thephloem• Translocation is the transport of organicnutrients in a plantMovement from SugarSources to Sugar Sinks• Phloem sap is an aqueous solution that is mostlysucrose• It travels from a sugar source to a sugar sink• A sugar source is an organ that is a net producerof sugar, such as mature leaves• A sugar sink is an organ that is a net consumer orstorer of sugar, such as a tuber or bulb• Sugar must be loaded into sieve-tubemembers before being exposed to sinks• In many plant species, sugar moves bysymplastic and apoplastic pathwaysLE 36-17Mesophyll cellCell walls (apoplast)Plasma membranePlasmodesmataCompanion(transfer) cellMesophyll cellBundle-sheath cellPhloemparenchyma cellSieve-tubememberProtonpumpLow H+ concentrationSucroseHigh H+ concentrationCotransporterKeyApoplastSymplast• In many plants, phloem loading requiresactive transport• Proton pumping and cotransport ofsucrose and H+ enable the cells toaccumulate sucroseLE 36-18Vessel(xylem)Sieve tube(phloem)SucroseSource cell(leaf)H2OH2OSucroseSink cell(storageroot)H2OPressureflowTranspirationstreamLE 36-19SapdropletAphid feedingStylet in sieve-tubemember (LM)StyletSevered stylet exuding sapSap dropletSieve-tubemember25
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