458 VENTILATING SYSTEM 



the far more active stomatic transpiration has to be taken into account 

 as well. 



Trustworthy and accurate quantitative data with reference to the 

 regulation of transpiratory activity by stomata, can only be obtained by 

 the method of weighing; but several investigators have achieved a 

 considerable amount of success with the aid of methods which are more 

 convenient, though less exact."" 4 Merget's plan is to place the experi- 

 mental leaf in contact with paper previously coated v/ith a mixture of 

 the hypochlorites of iron and palladium. This paper is yellowish-white 

 when freshly prepared, but gradually becomes darker as it absorbs 

 moisture ; the stain produced [in a given time] by contact with a 

 transpiring leaf will thus be darker or lighter accordingly as the rate 

 of transpiration during that time was high or low. Prints obtained in 

 this way, thus give some indication of the regulatory power of the 

 stomata. Stahl's " cobalt-method " is quite similar in principle, and 

 only diners in the nature of the substance employed as an " indicator." 

 Filter-paper is impregnated with a dilute (1 to 5 per cent.) solution of 

 cobalt chloride, a salt which is blue in the anhydrous state, but light 

 pink when hydrated. If a piece of blue cobalt-paper is placed upon 

 the stomatic surface of a freshly-picked leaf which should have been 

 exposed to sunshine just before it was gathered it frequently turns pink 

 within a few seconds, whereas similar paper ^placed in contact with the 

 astomatic surface of the same leaf may remain blue for hours {e.g. in the 

 case of Tradescantia zebrina, Salix caprea, Populus nigra, P. tremula, Pyrus 

 communis, etc.). The fact that this striking difference in the behaviour 

 of the two surfaces towards cobalt-paper can often be demonstrated in 

 the case of young leaves still enclosed in the bud, shows that the 

 cutinisation of the outer epidermal walls effectually restricts cuticular 

 transpiration at a very early stage of development. F. Darwin has 

 utilised the hygroscopic curvature shown by thin strips of specially 

 prepared horn (" Chinese sensitive leaf "), when placed in contact with 

 the stomatic epidermis of a leaf, in order to estimate the relative activity 

 of transpiration, and thus to determine whether the stomata are open 

 or closed. 



o. 



The relations of external conditions to the movements of stomata. 



It is obvious that the stomata would be incapable of regulating- 

 transpiration, and gaseous interchange in general, to the advantage of 

 the plant, unless their movements were determined by external condi- 

 tions. It would seem, above all, to be essential, that the guard-cells 

 should respond to any fluctuations in the intensity of illumination and 

 in the relative humidity of the atmosphere. As the width of the 



