CELL GROWTH AND DIVISION IN LIVING ROOT MERISTEMS

KNOWLEDGE CONCERNING the processes of growth and division of individuLal meristematic cells as they develop to maturity is clearly of importance for an understanding of the factors involved in cell differentiation and plant morphogenesis. Little is known about these processes in multicellular tissues except from fixed and sectioned material, the limitations of which are obvious. The most significant evidence can be obtained only by the use of living tissue wherein the same cell can be observed throughout its development. Recently a technique has been devised whereby growth and division of single cells in living root meristems can -be studied. Results obtained by the use of this method are reported in this paper. Sinnott (1939) has pointed out that the epidermal cells in root meristems of many small-seeded grasses can be clearly seen in the living condition since the root cap consists of but a few loose cells at the extreme tip. Seedlings are grown on microscopic slides covered with moist lens paper and kept in covered Coplin staining jars with a little water at the bottom of the jars. The growing seedling is flooded with water, covered with a supported cover glass, and placed under the microscope for observation of the growing root. By means of camera lucida drawings made at successive intervals of time, it is possible to observe cell division and growth. Using this technique, Sinnott and Bloch (1939a) made a study of root hair formation in Phleum pratense, Poa trivialis, Sporobolus cryptandrus, and Chloris gayana. It was found that in Phleum and Poa the last cell division is an unequal one which results in a small trichoblast or root hair-forming cell and a larger cell which does not form a root hair. The ,trichoblast is always toward the tip of the root and the hairless cell toward the base. During elongation, the trichoblasts grow less than the hairless cells. In Sporobolus and Chloris the divisions were found to be more nearly equal and both daughter cells may form root hairs. In a later paper (Sinnott and Bloch, 1939b), especial attention was given to "sliding growth" and differential wall growth, using Phleum and Sporobolus for observation. All the evidence obtained indicated that sliding growth does not occur. Differential wall growth was apparent in that: (1) parts of the walls of hairless cells opposite trichoblasts grow less than parts opposite other hairless cells; (2) under certain conditions, cells grow more in the middle portions than near the ends; (3) the end of a cell toward the base of the root attains its final size sooner than the apical end of the same cell. 1 Received for publication March 23, 1942. The author wishes to express his appreciation to Professor E. W. Sinnott of Yale University for criticisms and suggestions during the course of this investigation. This paper is part of a dissertation presented to the Graduate School of Yale University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. These observations show that the differences in size of large cells and small ones are a result not only of unequal divisions but also of differences in their growth rates. The present investigation was undertaken to determine the growth rates of single cells and their descendants as they pass from the meristematic condition to maturity, thus obtaining more