"As a child, learning to swim, I had not at first opened my eyes under water, more fearful perhaps of injury to my sight than drowning. When I realised that it was safe to do so it was, of course, a revelation. When Robin Hill introduced me to the Clark electrode (Clark 1956) it was much the same. The ability to undertake continuous measurement; to see immediately the consequences of experimental intervention is immensely liberating. Sadly, however, the apparatus then available was far from ideal. Following Ohmar Khayam's edict, Tom Delieu and I therefore sought to

"crush this sorry scheme of things entire and remould it nearer to the heart's desire"

I made pencil drawings on the backs of envelopes. Tom, a master craftsman, turned these into physical reality. Clark's electrode was a major achievement. We simply put it into a mechanical environment better suited to our purpose (Delieu and Walker 1972) and the world still awaits a device which will perform as well in all circumstances. Even so there was soon more demand for our apparatus which we could meet ourselves. Derek Bendall introduced me to John Humby, a manufacturer in search of a product. We had a need for someone to make us apparatus. So started a long and mutually beneficial cooperation.

Some years later, despairing of the availability of cheap and simple apparatus for measurement of photosynthesis by students, I asked Tom to make me a chamber which would accommodate a Clark-type electrode and a leaf-disc in the gas phase (Delieu and Walker 1981, Walker 1987).

To my surprise, it soon became clear that this crude device had potential as a serious research instrument. Computer programming by John McCauly and George Seaton and the combined efforts of Dave Thoday, Richard Poole and many others at Hansatech Instruments worked wonders. Measurements of Quantum Yield which once tookgiants of the world of photosynthesis many hours to perform could be done in as many minutes. So far as my own research was concerned, the facility to make simultaneous continuous recording of rates of O2 evolution and chlorophylla fluorescence emission, combined with a growing interest in oscillations (which cemented lasting friendship and co-operation with Agu Laisk) made it immediately apparent that that there was an anti-parallel relationship between these two signals (see e.g. Walker 1992a,b)" REFERENCES

DELIEU T, DA WALKER 1972 An improved cathode for the measurement of photosynthetic oxygen evolution by isolated chloroplasts. New Phytol. 71: 201-225

DELIEU T, DA WALKER 1981 Polarographic measurement of photosynthetic O₂ evolution by leaf discs. New Phytol 89: 165-175

DELIEU TJ, DA WALKER 1983 Simultaneous measurement of oxygen evolution and chlorophyll fluorescence from leaf pieces. Plant Physiol 73: 534-541

WALKER DA 1989 Automated measurement of leaf photosynthetic O₂ evolution as a function of photon flux density. Phil Trans R Soc Lond B 323: 313-326.

WALKER DA The measurement of rate of photosynthesis as a function of photon flux density and the significance and implication of these measurements. Techniques and new developments in photosynthesis research. Plenum Pub.Corp NATO ASI Series A. Vol 168: 331-340

SEATON GGR, DA WALKER 1990 Chlorophyll fluorescence as a measure of photosynthetic carbon assimilation. Proc Roy Soc B 242:29-35.

SEATON GGR, DA WALKER 1990 Measuring photosynthesis by measuring cholophyll fluorescence. In "Trends in Photosynthesis Research" (J Barber, H Medrano, MC Guerrero, Eds) Intercept Limited, UK pp289-304

WALKER DA 1992 Excited Leaves Tansley Review 36: New Phytol. 121: 325-345

WALKER DA 1992 Concerning oscillations Photosynthesis Research 34: 387-395

WALKER DA, GGR SEATON 1992 Validating chlorophyll fluoresence measures of efficiency: observations on fluorimetric estimation of photosynthetic rate. Proc R Soc B 249: 41-47

WALKER DA, RC LEEGOOD 1992 Photosynthesis and respiration: use of O₂ electrodes In Methods in Comparative Plant Ecology - a laboratory manual (GAF Hendry & JP Grime eds) Chapman & Hall, London pp136-140

WALKER DA, 1993 Polarographic measurement of oxygen In Photosynthesis and Production in a Changing Environment : DO Hall, JMO Scurlock,Hr Bolhar-Nordenkampf, RC Leegood & SP Long. pp 168-180

WALKER DA, 1997 "Tell me where all past years are"Photosynth. Res 51: 1-26