Page:Popular Science Monthly Volume 79.djvu/552

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an intensity that would facilitate water loss, the rate suddenly drops, with the stomata still open. The theoretical explanation offered for this break by Professor Livingston would assume that the outer walls of the jelly-like cells are coated with a film of water from which evaporation takes place and which is constantly supplied from the cell. When the evaporating power of the air causes a loss in excess of the rate at which the film may be renewed from the cell, the film breaks, and evaporation now may take place from the interstices of the walls only. If the wall of the cell were supposed to be of brick laid in mortar and coated with plaster, the plaster would correspond to the film and the mortar between the bricks to the water from which evaporation could take place after this "incipient drying," as it has been termed, has taken place.

Excessive water loss may proceed with or without the breaking of the film to a point where the turgidity or pressure of the cells is lessened, with the result that the leaf wilts. The wilting point is not a constant, but is mainly the product of the retentivity of the soil and of the evaporating power of the air, both of which may vary widely. The evaporating action of the air may be calibrated exactly at any time, and it is proposed by Professor Livingston that the standard of wilting point for a test species might be one of the most valuable expressions of the agricultural value of a soil.

The action of stomata inevitably comes up in any consideration of transpiration: the beautifully regular structure of these organs, and their delicate action, have led to some extremely fanciful interpretations of their self-regulatory mechanism. Time suffices only to say that the condition of the stomatal openings concerns not only transpiration but also photosynthesis and respiration, and any scheme of automatism for action in response to any one of these processes would at times be highly detrimental to the other functions. Of recent contributions to the physiology of these organs, Lloyd's consideration of the manner in which carbohydrates are drawn into the guard cells and are concerned in the making or loss of turgidity, and also his method of determination of the actual state of the stomata on a leaf at any moment by instantaneous fixation of a strip of detached epidermis must be reckoned to be of great importance. Frances Darwin has recently devised a porometer which measures the rate at which air may be pulled through a leaf from one surface to the other, thus obtaining a basis for the calculation of the average condition of the stomatal openings. Such refinement of methods and perfection of apparatus will permit a much more accurate calibration of leaf action than has been possible hitherto.

The enormous accumulations of water in the bodies of cacti and other succulents raise questions as to the part such liquid may play in the life of the plant and some observations to test the matter were begun