Phloem Anatomy in Two Species of Nicotiana, with Notes on the Interspecific Graft Union

Abstract

1. The structure and development of sieve tubes seem causally related to normal conduction in plants, to the movement of certain viruses, and to the formation of the union in grafts and wounds. 2. Stages in the ontogeny of phloem elements in tobacco differ little from those described in potato. Primary sieve tubes contain slime bodies, nuclei, plastids, and the usual cytoplasmic structures. The young elements accumulate neutral red and may be readily plasmolyzed. As the mature, the nuclei and slime bodies disintegrate, vital stains do not accumulate, and the cells can no longer be plasmolyzed. 3. Secondary sieve tubes have no slime bodies. As they leave the cambium, the nuclei may divide one or more times to form as many as four nuclei in the differentiating cell. Later these nuclei disintegrate, plastids develop, and the cells acquire the usual properties of mature sieve-tube elements. Obliteration follows the same course as in potato. 4. Vascular strands differentiated from callus parenchyma connect stock and scion within as few as five days after grafting. 5. Cambium arises within these strands; and by lateral extension a complete layer is soon formed, continuous with the cambiums of the stock and scion. 6. Orientation of the cambium initials depends upon that of the original vascular strands. Where the matching of stock and scion is good, the strands are short and straight, and the cambium approaches in form that of the normal stem. Where matching is poor, the strands are long and crooked, while the cambium initials lie in varied patterns. Grafts of this type are so rough that conduction may be hindered. When no incompatibility exists, they usually become smoother with time. 7. Formation of definitive callus occurs abnormally early along the line of union, especially on the side of the stock. Sieve-tube plastids in these callused cells are normal in form and numbers. 8. No irregularities in pitting of cell walls could be found along the line of union in the N. glauca-N. tabacum grafts. 9. Root initials, surrounded by parenchyma cells heavily stored with starch, characterized the line of union in several grafts.