548 THE POPULAR SCIENCE MONTHLY 



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 evapo- 

 ration 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 evapo- 

 rating power of the air, both of which may vary widely. The evaporat- 

 ing 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 instanta- 

 neous fixation of a strip of detached epidermis must be reckoned to be 

 of great importance. Frances Darwin has recently devised a poro- 

 meter 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 calcula- 

 tion of the average condition of the stomatal openings. Such refine- 

 ment 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 



