1897.] by a New Method. 305 



A paper-scale being fixed to the cork block serves to read off 

 the movement : when applied to leaves, for instance a hyposto- 

 matal leaf, the reading is zero on the upper side and varies 

 on the other side according to the state of the stomata. It 

 has been hitherto impossible to get any physical measure of 

 what the readings indicate ; from a large experience of various 

 leaves it may be said that 50° means an extreme amount of 

 transpiration, 30° fair degree, 10° a small degree. 



It is sometimes difficult to know how far the readings of the 

 hygroscope can be taken to indicate the state of the stomata. 

 Thus for instance in variegated leaves I get low readings on 

 the white parts, high readings on the green parts. Is this due 

 to partial closure of the stomata or to there being fewer stomata 

 on the white patches, or is it because the absence of chlorophyll 

 diminishes the yield of vapour ? If the method fails in cases 

 like this it is nevertheless applicable in broader, less critical issues. 

 For instance, all microscopic observations from the time of Mohl 

 agree that certain leaves close their stomata when they wither. 

 It cannot be doubted therefore that the drop in the hygroscope 

 readings in withering leaves corresponds with closure : the same 

 argument applies to the effect of darkness. By taking readings 

 on the uninjured leaf of Caltha and by looking at the stomata I 

 have convinced myself that there is a rough agreement between 

 the hygroscopic and microscopic methods. But I generally get a 

 low hygroscope reading even when stomata look quite shut. 



If we may believe that broad differences in hygroscope 

 readings represent differences in the openness of the stomata, 

 we must not assume that where the hygroscope stands at zero 

 there is absolutely no loss of vapour from the leaf. Thus there 

 is no doubt that leaves lose some water by cuticular transpiration, 

 yet the hygroscope always reads at zero on a surface free from 

 stomata. Also a leaf whose stomata are apparently shut as indicated 

 by the hygroscope, continues to lose weight by evaporation. 



Since the movement of the index depends on the difference 

 between the moisture of the air on the two sides of the horn 

 plate it might be supposed that a given surface would give very 

 different readings in damp and dry air. But practically this is 

 not so. I used a porous brick standing in a shallow layer of 

 water ; on the brick I placed a sheet of perforated zinc and on 

 this the hygroscope ; readings were taken in the dry air of the 

 laboratory, in the less dry succulent house and in the various plant- 

 houses, aud I got no very serious differences in the readings. 



Another source of error is that the horn plate is affected by 

 heat, i.e. it curves away from a hot surface. This makes it 

 difficult to experiment on the effect of heat, but otherwise is 

 unimportant. 



