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The senescence of leaf discs cut from mature B r~~s s e l s s p r o~~t leaves is retarded by a number of kinins. Other plant growth reg~~lators are without effect. When young expanding leaves are treated with Itinins the senescence processes appear to be accelerated. The ef-lect of N-6-benzyladenine on the structure of the cell organelles in young and old leaves has bee11 studied by electron microscopy. I t has been found that ill mature leaves the chloroplast is the organelle which senesces first and the breakdown is retarded by iV-6-benzyladenine. The mitocho~~dria are very stable. 111 young leaves iV-6-benzyladenine appears to over-stin~ulate the chloroplast, causing enlargement and excessive membrane synthesis.

The freeze-thaw gelation of hen's egg yolk low density lipoprotein (1.d.l.) has been examined by viscometry, agarose gel filtration, electron spin resonance spectroscopy, electron microscopy and differential scanning calorimetry. The data indicate that the core lipids, the hydrocarbon chains of interfacial lipids of 1.d.l. and the sulphydryl, imino, amino and tyrosine groups in the side chains of 1.d.l. peptides are not directly involved in the gelation mechanisms. The gelation is caused by non-specific aggregation due to cross-linking of peptides and/or phospholipid polar head groups of adjacent 1.d.l. particles. During freezing, interparticle contacts are formed and strengthened as transformation of solvent water into ice increases the concentration of the particles. Salt and sugar additives inhibit large increases in concentrations of 1.d.l. during freezing and prevent gelation by helping to increase solvation of interfacial regions of the 1.d.l. particles. Agents that induce large viscosity increases in the 1.d.l. sols before freezing may act by producing a local conformational change in 1.d.l. protein and/or disrupting lipid-protein bonds.

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