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Warm-up: Where would you expect to find stomata on pond lillies? How can aphids help us understand phloem function? Nutritional Symbiosis ! Bacterium that lives INSIDE aphid gut cells. (Intra-cellular BACTERIOCYTE). ! Aphid cannot make 10 essential amino acids. ! Phloem sap contains almost NO protein. ! Bacteriocyte makes amino acids for the aphid. Ultrastructure of a Bacteriocyte Nucleus in the center. Buchnera cells packed around it. Homeostasis in humans Use chemical (hormones) and electrical (nerves) mechanisms to regulate responses to environmental change. How do plants maintain homeostasis? Plant responses are primarily mediated by hormones Hormones Chemical messengers Produced in one part of an organism and released to have an effect on another part of the organism. Plant responses: tropisms ! Light: phototropism ! Day-length: flowering, seed germination ! Gravity: shoots grow up, roots grow down ! Temperature: growth Tropism = growth response to a stimulus Plants: exhibit indeterminate growth in regions called “meristems” Animals: determinate growth !P eriods of rapid cell division and growth (embryonic, juvenile periods). !A fter this, growth slows or stops Plant hormone: Auxin Primary auxin is IAA (indole-3-acetic acid) Plant hormones affect growth: apical dominance Tropism: Plant response to stimuli which is growth mediated Auxin effects