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IMPROVING CRITICAL THINKING-defined by one expert as "reasonable reflective thinking focused on deciding what to believe or do"has been a frequent topic in the writing of history educators at least since the early 1970s.2 Although articles and papers from the 1970s3 through the 1990s4 have suggested that history textbooks can be a suitable focus for instructional activities on critical thinking, more often history educators have argued that source documents beyond the textbook and, today, electronic sources are especially good media for helping students improve their critical thinking abilities.5 Indeed, a convincing case can be made for emphasizing critical thinking whenever the Internet is used in history classes, partly because of the inconsistent reliability of the sources that can be easily accessed electronically.6 Even if students are guided to reputable sources for research and writing activities in history classes, Avner Segall has pointed out that the current intellectual climate, heavily influenced by critical theory, deconstructionism, and postmodernism, The History Teacher Volume 37 Number

New experiments and insights extend current concepts to yield four integrated hypotheses on polarization. (1) In tissues that lack polarity, as in meristems and proliferating callus, short-distance diffusive movement of auxin between spatially separate sources and sinks can gradually induce polar transport of auxin and associated proton currents in the direction of the initially diffusive movement. (2) The formation of auxin at the shoot tip, on which maintenance of the plant's polarity depends, occurs by hydrolysis of auxin precursors such as IAA-myo-inositol, which readily hydrolyses (even non-enzymically) at rates that increase with pH, so that higher pHs arising in leaf primordia through auxin-proton cotransport can promote local formation of auxin. (3) The long axes of cells and the secondary wall banding of tracheary elements tend respectively to develop parallel and perpendicular to the direction of polar transport of auxin through tissue, especially vascular tissue; these orientations are mediated by the bioelectric current associated with polar auxin transport, through orientation of cortical microtubules transverse to the current. (4) Initiation of polarity in zygotic embryos is controlled by the direction of auxin movement in the surrounding parental tissues; in lower plants the exoscopic embryo's polarity is similar to that of the surrounding tissues, but in seed plants the endoscopic embryo's polarity is inverted as a result of physiological isolation, localised auxin breakdown, and suspensor formation.

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