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718 Europe and America, and though the most renowned instrument makers were allowed to examine and sketch the essential parts of the apparatus, yet they found it impossible to duplicate the instruments. It was frankly admitted that our craftsmen possessed tactile delicacy which could not even be approached. Requests have therefore been made by different Universities that my Laboratory should supply them with duplicates of instruments for the successful pursuit of the intricate researches originated in India. It is necessary to lay special stress on the point at this juncture, when the assertion, totally ignorant and unfounded, is being made that the country is incapable of making any great industrial advance.

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The importance of due combination of introspective and experimental methods and the necessity of long persistence in solving the most intricate problems is best illustrated by the efforts which culminated in my present discovery. The problem of the rise of sap in trees has baffled all inquirers for more than two hundred years. How is the water transported from the soil to the top of a tree which in the giant Eucalyptus reaches a height of 450 feet? Is the movement of sap due to physical forces or to some unknown activity characteristic of living cells? In deciding this question Strasburger applied poison and imagined that it did not in any way modify the movement of sap. The rise of sap could not therefore be due to the activity of living cells, a conclusion which found general acceptance. The advance of knowledge has always been blocked by the dictum of authority, and no one ventured to question the conclusion of one so eminent as Strasburger. All the efforts of inquirers were henceforth diverted to the discovery of some physical cause for which the most far-fetched theories have been put forward in reconciling speculation with facts which contradicted it. It took me twenty-two years to demolish the various false assumptions and establish the correct theory. Five series of investigation have been carried out during these years, each of which independently confirmed the new theory. For my first series of investigation undertaken more than twenty years ago, I invented an apparatus for the automatic record of the rate of suction by the plant. I was then able to show that so far from poison having no effect on the movement of sap, it caused a quick and permanent arrest of the ascent, the propulsion of sap being therefore due to the activity of living cells. Though a very definite and conclusive demonstration was given, yet the conservatism of science was so great that it had little effect on those committed to the old theory.

In spite of special pleading, the physical theory was found to be wholly unsatisfactory. The ascent of sap was next supposed to be brought about by the action of two mysterious forces, one of which pushed from below, and the other pulled from above. The push was imagined to be caused by “root-pressure.” In order to show that this was not essential, I cut off the root with the result that the rate of ascent was actually increased. The pull from above was supposed to be entirely due to the transpiration from the leaves. I next cut off all the leaves and coated the bare stem with impermeable varnish, the cut end of the stem being placed in water. The movement of sap still persisted, proving that the active tissue which maintains the movement of sap is not confined to the root or to the leaf, but exists throughout the length of the plant. By stimulating the lower or upper ends of the bare stem, I was able to make the sap ascend or descend at will. As in the movement of blood in some lower animals in which the heart takes the form of an elongated tube, a peristaltic action causes the movement of sap.

There are various physiological agencies which enhance the activity of the animal heart. Do they have similar effect on the plant as evidenced by the increased rate of propulsion of sap? No satisfactory evidence was at first available since no detector was known, which served as a visible indicator of the movement of sap.

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No one imagined that the leaf was such an indicator. Under drought, it begins to droop; after irrigation it becomes erect. The rate of movement of the rise of the leaf can thus be made to measure the rate of ascent of sap. The leaf-movement is, however, too slight for purposes of exact measurement. This drawback has been entirely removed by my simple device of the Optical Lever. The leaf is attached to one end of the lever by a thread; the fulcrum rod carries a small mirror from which is reflected a beam of