272 



Sir F. Darwin. 



length from about 25 to 40 mm. The sum of the lengths of the incisions 

 = 2437 mm. The thickness of the leaves was taken as - 38 mm., and since 

 each incision exposes two leaf-sections to the air, the total area of section 

 exposed by the experiment is 



2 x 2437 mm. x 0-38 mm. = 1852 mm. 2 = 18*52 cm. 2 . 



The stoma-bearing area of the 10 leaves, omitting the mid-ribs, was 

 600 cm. 2 , so that the amount of surface exposed by incision is 18 - 52 per 

 600 or 3'09 per cent. Unger* gives for P. laurocerasus the intercellular 

 spaces as 21 - 9, say, 22 per cent.f of the volume of the leaf. Therefore of the 

 transverse section exposed by incision only 22 per cent, is intercellular space. 

 We may therefore say that in a laurel leaf having four incisions on each 

 half of the lamina the transpiratory apertures connecting the intercellular 

 spaces with outer air are 22 x 3'09/100 or - 68 per cent, of the area of the 

 leaf. Since these correspond in function to stomata it is worth while 

 comparing them with actual stomata. 



A rough calculation gave the area of the laurel stomata as - 88 per cent, of 

 that of the leaf. The transpiring area of the slit leaves is, therefore, much 

 the same as that of the stomatal apertures under ordinary conditions. 



The Effect of Changes in the Humidity of the Air. 



The method of incision has been used in studying the effects, on transpira- 

 tion, of variations in the relative humidity of the air ; and this has led to a 

 rough plan for reducing transpirations at varying humidities to a common 

 standard. The method of producing a damp atmosphere was a simple one. 

 At first the plant was covered with a large bell-jar x'esting on a ground- 

 glass plate, and so arranged that a current of air, dry or moist, could be 

 drawn through it. But finally I came to the conclusion that a simpler 

 method was preferable, namely, to change the relative humidity by raising 

 or lowering the bell-jar ; in this way — assuming that the laboratory air is 

 fairly dry — it is easy to change the relative humidity from 50 per cent, 

 to 95 per cent., which is sufficient for my purpose.^ The wet and dry 



* ' Sitzb. K. Akad. Wien,' 1854, vol. 12, p. 367. 



t Microscopic examination of a transverse section led me to estimate the air spaces as 

 roughly 25 per cent. 



| It is unfortunate that these observations, with the exception of Experiment 8, were 

 not made in darkness or in constant light. The experiments which are most likely to be 

 vitiated by this fault are Nos. 3, 4, and 7. Experiment 4 might be expected to give an 

 especially bad result from the effect of dull light at the end of the experiment. But the 

 diagram, fig. 4, shows rather striking uniformity in the relation between transpiration 

 and humidity of air. In Experiment 3 the diagram is not very satisfactory in any case, 

 but omission of the last two readings (the ones under suspicion) would not alter the 



