Developmental and physiological studies of bracken (Pteridium aquilinum)

The application of 2,4-D to the frond of the bracken sporeling induces an abundant proliferation of roots at the apices of the rhizome and a temporary inhibition of rhizome development. The failure of these effects to be produced when a segment of the treated frond had been killed indicated that the basipetal transport of the herbicide occurs in living tissues. The number of roots induced and the inhibition of rhizome development increases with the dosage applied while the elongation of the roots is correspondingly depressed. There is evidence that the direction of translocation within the rhizome is dependent upon the position of the treated frond, movement being greater into that branch arising on the same side of the stem as that on which the treated frond is inserted. This relationship is attributed to the nature of the vascular connections. The induction of large numbers of root primordia at the rhizome apices was found to be the most characteristic of the histological effects. It has also been shown that the application of 2,4-D to fully expanded fronds of mature bracken produces injurious effects at the apices of the rhizomes. The effects were similar to those induced by this treatment in the young sporeling. Autoradiograms from sporeling fronds treated with 2,4-D containing radioactive carbon (C14) showed that the direction of translocation is largely determined by the stage of development of the frond and provided good evidence that the herbicide is translocated in association with food materials. Employing the same technique in a field experiment it was found that the amount of the tracer which moved into the rhizome was considerably greater from parts of the frond which were still immature. It is suggested, as a possible explanation of this result, that the penetration of the herbicide may be restricted by changes associated with the maturation of the frond.