Studies in the Diatom Genus Hantzschia 3. Infraspecific Variation in H. virgata

Infraspecific variation has been investigated in the marineepipelic diatom Hantzschia virgata using light and electronmicroscopy. Although the variation pattern in this species iscomplex and to some extent discontinuous, allowing the recognitionon phenetic grounds of four or five discrete infraspecific taxaincluding vars. virgata (= H. insolita), gracilis, leptocephala(= capitellata) and wittii (= intermedia), it is concluded thatto split the species would be imprudent. Variation has beenfound not only in characters such as valve outline and size,or stria density, but also in ‘qualitative’ characterssuch as raphe, fibula and girdle structure, characters whichhave often been assumed to be constant within species, generaor taxa of higher rank: previous generalizations about the usefulnessof certain characters in distinguishing taxa of a given rankare seen to have been incautious. Hantzschia virgata (Roper) Grun., diatom taxonomy, infraspecific variation     

Protoplast Rotation, Cell Division and Frustule Symmetry in the Diatom Navicula bacillum

Navicula bacillum frustules, like those of many other naviculoiddiatoms, exist in two forms, cis and trans, which occur in theratio 1 cis: 2 trans. During interphase the protoplast bearsa constant relation to the orientation of the hypovalve, suchthat the nucleus lies on the same side as the first-formed halfof the hypovalve. Immediately before mitosis the plastid andnucleus migrate around the cell until the nucleus lies on theopposite side from its interphase position, where it then divides.After cytokinesis the new valves form with their primary halveson the same side as the recently divided nuclei. This and the180° rotation of the protoplast during the cell cycle explainthe 1 cis: 2 trans ratio. Navicula bacillum Ehrenb., cell symmetry, diatom systematics, plastid structure, protoplast rotation     

THE COMPETITION BETWEEN BARLEY AND CERTAIN WEEDS UNDER CONTROLLED CONDITIONS

Chickweed (Stellaria media) is one of the commonest annual weeds on almost all soils, and forms 20–30 % of the weed herbage on the lower greensand soils of Woburn, Beds. It occurs in all crops in this area except on very acid soils.
When barley and chickweed are planted together, with abundance of water and nutrients for both, it is found that increasing the density of planting of the barley reduces the loss due to chickweed competition, but even with very close-planted barley, the loss caused by the weed amounts to nearly two-thirds of the total fresh weight and to four-fifths of the grain yields. This is much greater than with the two annual weeds previously studied (spurrey and mayweed). In a sparse crop of barley, increase in chickweed growth may reduce the barley to less than 10 % of its growth without the weed, while the chickweed itself is relatively little affected by the presence of the barley.
There is no evidence of any specific effect of the roots of the one plant on the other; they intertwine without any sign of attraction or repulsion between them.
Comparisons are made with other annual weeds previously studied, and it is suggested that the method adopted furnishes a means of assessing the relative effectiveness of these weeds in competing with barley.