Extension growth in Primary Cell-Walls with Special Reference to Hippuris vulgaris

A quantitative study of pit-field distribution in the longitudinalwalls of cortical parenchyma shows that pit-fields increasein number continuously during cell division and growth. A considerationof the data in relation to other information on pit-field distributionand to wider aspects of cell-wall extension suggests that pit-fieldmultiplication may be expected to be a characteristic featureof the growth of intercellular cell-wall surfaces in meristemcells.     

THE NORMAL FLORA OF THE BOVINE RUMEN II. QUANTITATIVE BACTERIOLOGICAL STUDIES

SUMMARY: A simple medium and dilution count technique was evolved for determining the viable count of rumen bacteria. Material from three fistulated cows was examined and the viable population found to vary within the range 10⁷-10¹²/g; the most frequently occurring values were 10⁸-10¹⁰/g (94.0% of 350 counts). Observations were made of the homogeneity of rumen contents, of variations in count with time after sampling and between cows, and of variation from day to day, hour to hour, before and after feeding and on different diets. The variations in the viable count, even with gross changes in diet, were never greater than those occurring in a single animal on a fixed diet. Microscopic examinations showed Gram-positive cocci and rods to be the most frequently occurring types of bacteria, Gram-negative organisms of the coli-aerogenes type being present only in low numbers.     

Selection for pollination success and the mechanical fit of Imfiatiens flowers around bumblebee bodies

Selection on flowers has often been viewed as being particularly strict, constant, and responsible for species differences. Impatiens pallida and I . capensis flowers fit snugly around bees, leading one to expect a close relationship between floral morphology and pollination success. My studies on the amount of pollen removed from androecia and deposited on stigmas in single visits by bumblebees did not confirm this supposition. Trimming off parts of the floral vestibule with scissors and gluing in pleats had very little effect on die amount of pollen that bees moved. In reciprocal transfer experiments, flowers from different populations sometimes differed in the amount of pollen moved, but when the two species were compared in sympatry, pollen removal and deposition differed hardly at all. A comparison of the relationship between pollen movement and floral morphology among 15 populations showed that, although there was great heterogeneity in die amount of pollen moved, the observed differences were independent of floral morphology. None of this supports a belief in strong selection that fine-tunes the mechanical fit between bee and flower; selection for visitation success based on pollinator behaviour may have a much stronger influence on floral characters.