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 sto...