The Comparative Morphology and Development of Isoetes L. II. Branching of the Base of the Corm in I. tuckermanii A. Br. and I. nuttallii A. Br.

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Abstract
Branching of the root-bearing portion of the corm of Isoetes is reported for the 1st time. The specimens range in age from about 3-36 yr and show a wide variety of structural variations. The main characteristic shared by all of the branched plants is the absence of a typically constructed rhizophore stele and root-producing meristem and the presence of branch steles that bear roots along their sides and in some cases also at their distal ends. The meristematic regions which give rise to the roots at the ends of the branch steles of some of the plants are very similar to the root-producing meristems of unbranched plants. Examples of both 1st- and 2nd-order dichotomies occur; 2nd-order dichotomies are cruciate. Branching of the root-producing meristem is further evidence that it should be regarded as a primary meristem. The axis pf Isoetes is therefore interpreted as being bipolar. This condition was also present in fossil forms such as Stigmaria, Pleuromeia, Nathorstiana and Nathorstianella. In constrast, the axis of the mature plant of Stylites is monopolar with a single growing point.

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