The Comparative Morphology and Development of Isoetes L. III. The Sequence of Root Initiation in Three- and Four-Lobed Plants of I. tuckermanii A. Br. and I. nuttallii A. Br.

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Abstract
Root initiation was studied in 2-, 3- and 4-lobed specimens of I. tuckermanii and I. nuttallii. The growth of root primordia in the cortex of the corm was synchronous until they emerged from the cortex; therefore, the lengths of the primordia are exact indicators of the time at which they were initiated. Lengths of primordia were determined from serial cross sections of corms. Series of roots may be drawn in only one way so as to contain an orderly sequence of initiation within each series. If the series are drawn so as to curve gently at both ends toward the root-producing meristem, then the sequence of initiation in a series begins at or near the center of the plant and proceeds on every orthostichy to either side of the original member of the series. In a few plants initiation within the series proceeded in a perfectly symmetrical sequence, but in most plants various degrees of asymmetry were seen in the sequence within the series. On multilobed plants, new series are added in a spiral which may turn in either direction. The general sequence of initiation within series is the same in plants of all ages, including the youngest sporelings.

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