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STOMATAL REGULATION 167 



from the leaf and instantly plunged in absolute alcohol, 

 the stomata retain their size and shape as in the fresh state, 

 and can be accurately measured (the average of a consider- 

 able number being taken) when convenient. This method 

 is due to Lloyd (1908). Darwin and Pertz (191 2) devised 

 an instrument, called the porometer, which has the advantage 

 that it may be modified to give continuous readings, or 

 records (Laidlaw and Knight, 191 6). It consists in essence 

 of a small glass funnel, the wide end of which, about i cm. 

 in diameter, is glued to the surface of the leaf. The other 

 end is connected to a vertical tube of water. The weight 

 of the water column draws air through the stomata, and 

 the rate at which this happens can be ascertained from the 

 time it takes the water column to fall a given distance. 

 Since, in the porometer, air is drawn through the stoma, 

 the rate of its passage is proportional to area and not to 

 diameter, as with diffusion. For this reason the square 

 root of the time is taken as inversely proportional to the 

 aperture of the stomata. The amount of transpiration can 

 be very accurately determined by weighing ; if a potted 

 plant is employed, the pot and earth are covered with a 

 suitable waterproof case. Self-recording apparatus has 

 also been employed here. The rate of absorption of water 

 from a glass tube {potometer) has also been employed as a 

 measure of transpiration, but, except in conditions where 

 absorption just balances loss, is obviously less accurate. 



The results obtained by the use of these methods have, 

 on the whole, been unfavourable to the hypothesis that 

 stomatal movement is the most important mode of regulation 

 of transpiration by the plant. Following the rate of tran- 

 spiration of the leaves and the changes in stomatal aperture 

 for Verbena ciliata through the 24 hours, Lloyd (1908) 

 showed that, under normal laboratory conditions, the 

 transpiration rate, which was low at night, began to rise at 

 about 2 a.m., reaching a maximum at between 10 a.m. 

 and II a.m. Before noon it began to fall, and this fall 

 continued, with a slight check in the afternoon, till mid- 

 night. The stomata begin to open about 2 a.m., and become 



