Studies onAzotobacter species in soil

Summary Methods for countingAzotobacter species in soil have been examined. The highest counts were obtained from soil suspensions shaken in sterile distilled water containing 10-g glass beads and plated on to glucose agar. Mannitol has been rejected as a suitable substrate in agar media because it gives lower counts of Azotobacter than glucose, an effect which is further enhanced by drying the agar plates. A clear medium free from precipitated phosphate and CaCO₃ is recommended for the agar-plate method; the Azotobacter count is affected by the phosphate concentration.The agar-plate and dilution-tube methods were compared; the latter is less accurate but more convenient when many soil samples have to be examined.     

Regulation of Mitosis in Living Cells. I. Mitosis under Controlled Conditions

A quantitative method has been devised to study mitosis in vitro by phase contrast and polarization microscopy. Mitosis in cell-wall-free endosperm cells of Haemanthus kathrinar Baker (the African blood lily) has been divided into 18 arbitrary stages or events. The time course for the various stapes, as well as the percentage of cells that proceed from one stage to another during a four hour observation period, are presented. Cells that were in prophase when selected for study proceeded from nuclear membrane breakdown to melaphase in 60 minutes and remained in melaphase for 30 minutes. Only 13 minutes was required to proceed from onset of anaphase to mid-anaphasc. Mid-anaphase provides a clear and precise baseline for determining the time required for succeeding stages to appear. The cell plate made its appearance 40 minutes after mid-anaphase and was completely formed 20 minutes later. The nuclear membranes also became evident at this latter time and nucleoli were visible 30 minutes later. Thus, the average time for a cell observed initially in prophase to proceed from nuclear membrane breakdown to formation of two daughter cells was just over three hours. A high percentage of cells that were in late prophase or later stages of mitosis at the time of initial observation completed mitosis during the observation period. The effect of the length of time a cell is subjected to experimental conditions upon its subsequent behaviour is assessed. These results form the basis for future studies of the effects of chemicals, particularly herbicides, upon cells in mitosis as observed in vitro by phase contrast and polarization microscopy.     

A STUDY OF MEIOSIS IN HAPLOPAPPUS GRACILIS (COMPOSITAE)

Jackson , R. C. (U. Kansas, Lawrence.) A study of meiosis Haplopappus gracilis (Compositae). Amer. Jour. Bot. 46(7): 550–554. Illus. 1959.—A study of meiosis in the two-paired Haplopappus gracilis has shown that each pair of chromosomes is easily recognized throughout the various stages of meiosis beginning with pachytene. A comparison of mitotic and meiotic chromosomes has been made, and the data indicate that a conspicuous heteropycnotic knob on chromosome B at pachytene is the nucleolar organizer. Other recognizable chromomeres were observed, but additional study is needed before a cytological map can be drawn to show whether regions other than those described on chromosome A and B may be used as markers.     

RECURRENT CARCINOMA OF THE RECTUM—Surgical Treatment

Following operations on the rectum for carcinoma, approximately half of the patients have recurrence in the perineum, pelvis, abdomen or at the suture line of anastomosis. The prognosis is almost uniformly poor and although the problems of management are complicated, dealing with them may give the patient worthwhile physical, emotional and economic benefits. Surgical procedures used in the treatment of the common types of recurrence are discussed.