Abstract
Tracheid diameter, wall thickness, and wall area per tracheid increased towards the root tip in first-year seedlings of Picea sitchensis and Pinns sylvestris ; this is shown to be associated with differences in duration of the expansion and wall thickening phases of tracheid development. Transfer to short days produces a similar response throughout the plant, tracheid diameter and wall thickness decreasing shortly after transfer, due to a decline in rate of development. These data are discussed in relation to growth-regulator gradients likely to exist between root and shoot. It is pointed out that though effects of growth regulators on tracheid dimensions are well documented, it has yet to be established whether this is due to their regulation of the rate or of the duration of development.
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