110 LENTICELS. [CH. IV 



(123) The effect of water on stomata 1 . 



The majority of stomata close when surface-sections 

 of the leaf are placed in water. Some leaves however 

 behave in the reverse way : of these the most easily 

 accessible are those which form the floating rosettes of 

 Callitriche, If the tissue of the lower surface of the leaf 

 is gently scraped away with a needle, and the leaf is 

 mounted in water with the upper surface upmost the 

 stomata are visible ; they can be made to close by 

 irrigation with 2'5 p.c. NaCl solution, and again to open 

 by replacing the salt solution with water. 



(124) Electric effect*. 



Strips from the under-surface of the leaf of Ranunculus 

 ficaria are mounted dry under a cover-glass, on a slide 

 bearing a pair of microscopic electrodes. On passing the 

 induced current the stomata close. A current, slightly 

 stronger than that bearable on the tongue, is necessary. 

 The closure of the stomata may also be observed with the 

 horn-hygroscope 3 . 



(125) Lenticels*. 



The fact that lenticels communicate with intercellular 

 spaces may be conveniently studied in connection with the 

 parallel results obtained with stomata. Fit a woody 

 dicotyledonous branch (dog-wood, Cornus sanguinea, does 

 well) to the short arm of a U tube by means of firmly wired 



1 For this and other points see Mohl, Botan. Zeitung, 1856; Leitgeb, 

 Mittheilungen Bot. Institut Graz, 1886. 



2 N. J. C. Miiller, Pringsheim's Jahrb. vin. 



3 Francis Darwin, Phil. Trans. B. 1898, p. 567. 



4 Stahl, Botan. Zeitung, 1873, p. 613 ; the author recommends Gingko 

 biloba, Sarnlucus nigra and Lonicera tatarica. 



