Mitosis in zoosporangia of the chytrid Phlyctochytrium irregulare is described from electron microscope observations and also from light microscope observations of both living and haematoxylin-stained thalli. At the onset of prophase the centriole complex replicates, and the complexes migrate to polar positions. The semi-persistent nucleolus is appressed to the nuclear envelope as the nuclear pockets invaginate, finally rupturing to create polar fenestrae, through which spindle microtubules penetrate the nucleus from the region of the centrioles at prometaphase. Metaphase chromosomes form an equatorial plate. Initial separation at anaphase seems to be accomplished mainly by shortening of chromosome-to-pole microtubules; additional anaphase and telophase separation is accomplished by elongation of the nucleus. A system of perinuclear endoplasmic reticulum is formed during prophase and is completed by metaphase. It persists during all division stages after its formation. Features of this mitotic apparatus are discussed with reference to earlier light microscope studies of chytrid mitosis. The ultrastructure of P. irregulare's mitotic apparatus is similar to that of certain unicellular green algae.

We examined cell length, mitosis, and root meristem "cuticle" in different tissues of geostimulated, red light-exposed primary roots of corn (Zea Mays, Wisconsin hybrid 64A x 22R). The examination was done at 15-minute intervals for a period of 240 minutes. Differences in cell elongation between the upper and lower sides were most prominent between 1.5 and 2.5 mm from the root meristem; the outer cortex had the greatest elongation growth, and the upper cells showed a significant increase in length compared to the lower. A differential mitosis was also found, with the lower tissue being greater. We infer that the mitotic activity is indicative of cell division, and this division occurs strictly in the first 1.5 mm of the root meristem. The combined effect of differential cell elongation and cell division results in the localization of the geotropic curvature in the 1.5- to 2.5-mm region from the root meristem. Mitosis that occurs primarily in the cortex and stele were asynchronous; the peak of cortical division preceded that of the stele. Both peaks occurred before the peak of geotropism. A densely stained layer separates the cap from the root meristem. This layer is thinner at the apex of the root meristem. The area of the thin region increased with time and peaked at 180 minutes after geostimulation, which was coincidental with the peak of the geotropic response.

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In outer cortical cells of corn (Zen mays L.) roots we made ultrastructural comparisons between the geotropicafly responding (661 m irradiated) and nonresponding (dark control) roots in both the curving and noncurving regions of the root. In the control treatment, Golgi apparatus (dictyosomes) and mitochondria exhibited centrifugal distribution (taking the stele as the center) in both regions of the roots (the organeUes localized in the top of the ceUs in the upper tissues, and in the bottom of the ceUs in the lower tissues). In the geotropicaUy responding roots, the distribution patterns were the same as those of the controls. However, in the zone of curvature the dictyosomes (but not the mitochondria) were randomly distributed in the cels of the upper tissues. This change in pattern of dictyosome distribution could be related to the change In cell elongation of upper cels.In a previous paper (6), we compared ultrastructural differences in root cap cells of maize between the geotropically responding, red light-exposed roots, and the nonresponding dark control roots. The root cap is the site of perception in the geotropic response. In a light microscopy examination (8), we established that the curving zone of geotropically responding corn roots is located in the 1.5-to 2.5-mm region from the root meristem, or 2 to 3 mm from the tip of the root. The outer cortical cells of corn roots, the row of cells next to the epidermis, exhibit the greatest elongation among all root cells (8). In the present study, we have compared cell ultrastructure between the geotropically responding and nonresponding corn roots in the outer cortex of the curving regions and in regions behind the curving zone of corn roots. MATERIALS AND METHODSPlanting and Irradiation. The detailed procedure can be found in reference 6. Corn seeds (Wisconsin hybrid 64A x 22R) were soaked in warm water, drained, and refrigerated overnight, and planted on both edges in Lucite bars wrapped with moist filter paper. The bars were kept moist and away from light. The primary roots grew to a 1.5-to 3-cm length within 48 hr of planting, and extended horizontally from the embryo. These roots were used for experimentation. The horizontal pattern of growth subjected the roots to a constant gravitational stimulation during the entire period of growth and experimentation. As long as the roots were not exposed to light, they remained horizontal and showed no By acceptance of this article, the publisher or recipient acknowledges the U.S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering the article.' Work supported by the U.S. National Aeronautics and Space Administration and the U.S. Energy Research and Development Administration.geotropic curvature. A red irradiation (661 nm at I J m-2) would induce them to curve toward gravity (Fig. 1), i.e. becoming positively geotropic (7). Thus, all manipulations were carried out in darkness before the irradiation, except for seed planting which was done in a dim green light...

Te root cap is the site of graviy percepton. In the study of caps of primary roots of corn (Zea mays L), we compared the utrastructure of geotropicaly r n g roots that had received a 661 -(red) imriation (60 second) with dark control roots kept In the dark, at com rable times folowing geostubt for a total of 150 mIntes. lTe outaidifferences In the liht-exposed root caps at the ultrastructurd level were (a) siiflcandtly more Gol ratus (dictyosomes) were found in the top thas In the bottom of red-exposed cells; a random distribution IS seen In the dark control cells; (b) the nuleus preferred the top in a greater number of the red-exposed cells; (c) the pattern of mitochondria locaizati was identical In both treatments, a greater preference for the top, however, the nmber of mitochonra was reduced In the bottm of red-treated cap ces as ma ed to the control cells. A lowering In number In the bottom of the red-treated cells was noted also in the dictyosomes; and (d) in a small percentage of cells that sbowed a preferential distribution of eos reticulm (ER), more red-exposed cells than controls, durig the period 30 to 135 nutes after sdtulaton, had less ER In the top; bowever, a majority of the cels in both treatments sbowed no preferred position for ER ditribution. Comon is in ultrastructural behavior also existed between the red-and dark-treated root cap cells: (a) sedentation of a ts, ith no erence In total number between treatMents; and (b) a clse associatin between amyloplasts and ER In both groups.Polarization of organelles occurred in both the geotropically re ig and nonrponding roots. Te dffeences in dictyosome and nuclear localzation, and dictyosome and mtochodrial number could be correated with the topi response in the red-exposed roots and no respone in the dark roots, which in turn could be reated to tie reported hormoal events in the geotropism of roots.In the study of geotropism, investigators have looked for organelles in plant cells that are responsible for the perception ofgravity.The amyloplasts have been the long held candidates for the sensor, or statolith (14). However, there is no evidence to suggest a causal relationship between the movement of amyloplasts and the perception of gravity in the geotropic response of plants. In the geotropic study of oat shoots, we found that the Golgi apparatus (dictyosomes) responded rapidly in orientation and activity after gravity stimulation (25). A change in mitochondrial distribution