Factors Limiting the Stimulation of Polyploid Mitoses in Intact Pea Roots and Excised Root Segments

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Although polyploid mitoses can be elicited in pea root segments 1 mm in length cultured on a defined medium, the same medium failed to elicit any polyploid mitoses when applied to the roots of intact pea plants. Auxin treatments, which are effective in eliciting polyploid mitoses in the roots of other plants, were also ineffective when applied to pea roots. Polyploid mitoses were not obtained when excised whole pea roots were cultured on a medium which effectively induced such mitoses in 1-mm segments, nor were polyploid mitoses found when the roots were fed basally with this medium. Polyploid mitoses were observed in 1-mm root segments split in half along the axis of the vascular cylinder and cultured with the cut surface against the surface of the medium so that the vascular cylinder was oriented parallel to the surface of the medium. If these segments were turned so that the vascular cylinder was still oriented parallel to the surface of the medium, but so that the cut surface was up and the intact epidermis in contact with the culture medium, no polyploid mitoses were observed. Thus wounding alone does not initiate polyploid mitoses. The appearance of polyploid mitoses requires the presence of a cut surface in contact with the culture medium. Root segments 3 mm in length were cultured with one cut surface against the surface of the culture medium and the vascular cylinder oriented perpendicular to the culture medium. After 74 hr in culture the segments were cut into three 1-mm pieces-top, middle, and bottom. A significant number of diploid mitoses was observed in all three pieces, although there were fewer mitoses in the top piece than in the bottom. A significant number of polyploid mitoses was observed only in the bottom piece. The orientation of the 3-mm segments (basal or apical end down) had little effect on the results. Thus not only does the appearance of polyploid mitoses require that a cut surface of the root be in contact with the medium but polyploid mitoses are also elicited only within 1 mm of this cut surface. These facts make the failures to elicit polyploid mitoses in excised roots and roots of whole plants easily understandable; that is, the necessary stimulatory substances presumably never reached the root tissues. Wounding of the intact root by passing a needle through it stimulates mitoses of both diploid and polyploid cells. We suggest that interference of the normal flow of hormonal materials, probably of a cytokinin, from the root tip toward the root base (shoot) is responsible for the local accumulation of stimuli leading to polyploid mitoses.