Ontogenetic and Histochemical Changes in the Vegetative Shoot Tip of Chenopodium album

Gifford, Ernest M., Jr., and Herbert B. Tepper. (U. California, Davis.) Ontogenetic and histochemical changes in the vegetative shoot tip of Chenopodium album. Amer. Jour. Bot. 49(8): 902–911. Illus. 1962.—A distinct central zone is present in apices of young, rapidly growing seedlings of Chenopodium album. This zone is clearly evident when the plane of section includes developing leaf buttresses and/or newly formed leaf primordia, but is not visible if the cells associated with these developing foliar organs are not in the plane of section. Cells in the central zone contain markedly lower concentrations of RNA than those of the peripheral zone. Differences in DNA, SH-protein, total protein, and histone concentration of cells in the apex parallel the zonal distribution of RNA, but are much less obvious. As the plant ages and the rate of leaf production decreases, the zonal distribution of the above-mentioned compounds becomes less and less obvious. Cells at sites of leaf initiation in older seedlings still have slightly higher RNA concentrations, but a central zone such as is visible in the young seedlings is no longer evident. The axial tunica cells do, however, contain noticeably lower DNA and histone concentrations than the remaining cells of the shoot apex. Changes occur in the size of nucleoli during ontogeny and the possible relationship between nucleolar size and modifying factors is discussed.