THE HUMAN CHROMOSOME : Electron Microscopic Observations on Chromatin Fiber Organization

Human lymphocytes were grown in short-term tissue culture and were arrested in metaphase with Colcemid. Their chromosomes were prepared by the Langmuir trough-critical point drying technique and were examined under the electron microscope. In addition, some chromosomes were digested with trypsin, Pronase, or DNase. The chromosomes consist entirely of tightly packed, 240 ± 50-A chromatin fibers. Trypsin and Pronase treatments induce relaxation of fiber packing and reveal certain underlying fiber arrangements. Furthermore, trypsin treatment demonstrates that the chromatin fiber has a 25–50 A trypsin-resistant core surrounded by a trypsin-sensitive sheath. DNase digestion suggests that this core contains DNA.     

Root Formation by Detached White Mustard (Sinapis alba) Cotyledons

Rooting is shown to occur in excised cotyledons of Sinapis alba when grown in petri dishes on moist filter paper. Cotyledons were excised at intervals from 6 hours after the start of imbibition, when they were yellow, unexpanded and enclosed within the testa, to 27 days after sowing when the cotyledons were green and fully expanded and on plants possessing up to 3 foliage leaves. Rooting generally began 5 or 6 days after excision and was completed dining the following 5 days. The age of cotyledons at t ho time of excision had three effects on rooting: the lag period be-fore rooting began and the period during which rooting took place both increased with age: but the most marked effect was on the total number of cotyledons which were able to form roots, which increased until cotyledon expansion was almost complete, then decreased as the mature cotyledons became older. Optimal rooting was shown by cotyledons detached 8 days alter sowing, when they were half expanded. At this age S5 % of them formed roots between 6 days and 8 days after excision.     

Influence of Calcium and Magnesium on Manganese Absorption

Mutual effects between Mn, Ca, and Mg were studied during steady-state absorption experiments with excised barley roots. Calcium appeared to enhance the rate of Mn absorption; whereas, Mg had a highly depressive effect. The combination of both Ca and Mg was even more inhibitory to Mn absorption than Mg alone. Manganese had no effect on the usual negligible Ca absorption by this tissue, but effectively inhibited the absorption of Mg. Although divalent cation absorption from the Ca-Mg-Mn system was essentially nil, K absorption was greatly stimulated in the presence of these cations.These mutual effects and others reported in the literature are explained by the hypothesis that selectivity in ion absorption results from cation-induced conformational changes in the structure of the carrier molecule.     

QUANTITATIVE CHANGES IN GIBBERELLIN AND RNA CORRELATED WITH SENESCENCE OF THE SHOOT APEX IN THE ‘ALASKA’ PEA

Senescence of shoot apices of Pisum sativum L. ‘Alaska’ as measured by cessation of stem elongation was delayed by removal of flowers and by treatment with gibberellin A₃ and was hastened by treatment with AMO-1618 (2 isopropyl-4-dimethylamino-5-methylphenyl-1-piperi-dinecarboxylate methyl chloride). Ontogenetic changes in relative endogenous gibberellin levels and in capability of gibberellin biosynthesis in deflowered and control plants were determined indirectly by studying time-course changes in the sensitivity, as indicated by the growth response, of these plants to applied gibberellin and AMO-1618. The results of these experiments suggest that the endogenous gibberellin level varies directly with the growth rate. Analyses of total RNA and protein in shoot tips of deflowered and control plants revealed that the levels of these substances also vary directly with growth rate throughout ontogeny. It is concluded that decreases in endogenous gibberellin, RNA and protein are factors correlated with senescence of the shoot apex.     

Manganese absorption by excised barley roots

Short-term absorption studies with 5-day-old excised barley roots revealed that the basic aspects of Mn absorption were similar to those of other metabolically absorbed cations. Following an initial non-metabolic equilibration with the root, Mn was absorbed for several hours at a slower steady-state rate comparable to that of other inorganic cations. Complete or nearly complete inhibition of the steady-state phase by low temperature, dinitrophenol, and azide provides strong evidence that Mn transport into this tissue was metabolically mediated. Within limits, the rate of transport was strongly dependent upon the concentrations of Mn and the hydrogen ions in the ambient solution. Absorption increased rapidly with increasing concentrations of Mn up to 1 meq per liter. Above this concentration, the rate leveled off, apparently due to a saturation of the transport mechanism. Within the physiological pH range in which Mn is soluble (below pH 7), absorption increased greatly with decreasing hydrogen-ion concentration.