100 YEARS AGO

The structure of the hairs of the Patagonian ground-sloth and of the living South American edentates forms the subject of an essay by Dr. W. G. Ridewood, which appears in the May issue of the Quarterly Journal of Microscopical Science. The most generally interesting of the author's observations are those relating to the hairs of the two living types of sloth, and the structure which permits of the growth of an alga in each. In the three-toed sloth the hair is invested with a thick extra-cortical layer. “The layer has a tendency to crack in a transverse direction, and in the cracks there come to lodge unicellular algae, to which Kühn has given the name Pleurococcus bradypi. The moisture of the climate in which Bradypus lives enables the alga to live and propagate in this curious position, and the sloth acquires a general green tint which must render it very difficult to distinguish as it hangs among the green foliage.”

From Nature 20 June 1901.

50 YEARS AGO

One of the widely accepted tenets of plant physiology is that photosynthesis is intimately associated with the integrity of green cells. Disruption or even injury to the intact cell leads to the cessation of the process. This view has been based on the failure, for almost three-quarters of a century, of the many attempts to reproduce photosynthesis outside the living cell. The early experiments have often sought to attain an objective which has been aptly characterized as alchemistic in the light of modern knowledge: the performance by broken or dead cells or even by chlorophyll solutions of the process of photosynthesis in toto, that is, the utilization of light energy for the reduction of carbon dioxide and the simultaneous evolution of oxygen. However, these early attempts were not wholly negative. Although complete photosynthesis was never reproduced in vitro, some of the early experiments of Haberlandt, Ewart and Molisch indicated that certain cell preparations retain a limited capacity for oxygen evolution under the influence of light. The full significance of these observations did not become apparent until Hill showed by independent biochemical methods that chloroplasts removed from living cells retain for an appreciable period of time that part of the photosynthetic apparatus which is responsible for the evolution of oxygen.

From Nature 23 June 1951.