Two earlier reports' from this laboratory seemed sufficiently to establish the point that a film of Bordeaux mixture increases the rate of transpiration of excised leaves of the castor bean, and likewise of potted tomato and potato plants. It was further indicated that other surface films, and to a slight extent dusts, may produce a similar, though on the whole lesser, accelerating effect. Nevertheless, it was felt that this work left incompletely answered questions relating both to the immediate and final effects of the spray, as well as to the relation of the acceleration found to night and day conditions, or to changes of environmental conditions in general. Again, it seemed eminently desirable to construct and employ in this work a rotating table, or transpirotaplane, by means of which it might be hoped to eliminate such errors as might arise from differing transpiration rates induced through the position of the plants in the different intervals, whether the plants observed were in the greenhouse or in the open. In the earlier experiments it had not been possible to take sufficient cognizance of these points, but immediately upon the presentation of the reports referred to above, further experiments were begun leading in the directions indicated. A variety of considerations prevented a prompt conclusion of this work. Meanwhile, there has appeared a paper by Martin2 affording interestingand definite