Cell Division In Meristems Research Articles

IN SEARCH OF A PLANT GROWTH PARADIGM

Growth by cell divisions in apical meristems is mainly a synthesis of cell matter. Cell divisions in the meristem follow certain patterns, with radial gradients which may be abruptly interrupted. The gradients go in the reverse directions in stem and root apex. The cause of the gradients seems to be unknown, likewise why the patterns are the opposite in stem and root, but they form the foundation of ensuing tissue differentiation. Cell elongation is driven by the metabolic energy of intussusception of new matter in the wall and by the accepted patterns for water fluxes. An attempt has been made to formulate questions which ought to be answered for a better insight into the mechanism of longitudinal growth.

Ultrastructure of the corallinaceae. I. The vegetative cells of Corallina officinalis and C. cuvierii

A technique utilizing combined fixation and gentle decalcification has been employed to study the ultrastructure of the vegetative cells of the articulated calcareous coralline algae Corallina officinalis Linnaeus and C. cuvierii Lamouroux (Rhodophyta: Cryptonemiales). The epidermal cells are distinctive, with many cell wall inggrowths which pass between the chloroplasts. It is suggested that these cells function as “transfer cells”. The epidermal cells contain no starch, although the chloroplasts have well developed photosynthetic lamellae. Damage to these epidermal cells leads to formation of new cells by renewed division of sub-epidermal meristematic cells. The outer cortical cells have few small vacuoles and many plastids, with an extensive photosynthetic lamellar system. Deeper into the thallus, the vacuoles increase in size and free cytoplasmic starch grains occur. The medullary cells have a very large vacuole and in older tissue often appear dead. The long genicular cells have calcareous walls at either end while the wall in the middle of these cells is non-calcareous and has an inner fibrillar layer and a thin outer “cuticle”. In partially decalcified material, the orientation of the CaCO3 (calcite) crystals next to the cells can clearly be seen. Immediately next to the cell the crystals are fairly small and arranged at right angles to the plasmalemma. Further away from the cell the crystal size is larger and their orientation is more random. The crystals are surrounded by organic material, and the possible role of this material in calcification in coralline algae is discussed.

Division, expansion and DNA synthesis in meristematic cells of French bean ( Phaseolus vulgaris L.) root-tips invaded by tobacco ringspot virus

The division, expansion and DNA synthesis of meristematic cells of the root-tip of French beans were examined at different times after infection with tobacco ringspot virus. At about the same time that the terminal millimetre of the root was invaded by virus there was a drop in DNA synthesis, followed by a decrease in the mitotic index to about half that of healthy tissue. Cell elongation was unaffected, resulting in a gradual accumulation of expanded cells in the root-tip. Subsequently, the mitotic index of the root-tip returned to a normal level.

CELL GROWTH AND DIVISION IN LIVING ROOT MERISTEMS

KNOWLEDGE CONCERNING the processes of 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 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 growth and differential wall growth, using Phleum and Sporobolus for observation. All the evidence obtained indicated that sliding does not occur. Differential wall 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 rates. The present investigation was undertaken to determine the rates of single cells and their descendants as they pass from the meristematic condition to maturity, thus obtaining more