24 SCIENCE PROGRESS. 



inevitably leaks or diffuses in from the external air during 

 an experiment of twelve hours' duration. 



Allowing for these increments, it may well be that 

 through an absolutely uninterrupted cuticle, no C0 2 at 

 all diffuses if there be an open alternative route through 

 the stomata. We are certainly justified in concluding that 

 the stomata are practically the sole path for exhalation of 

 respiratory CO,. 



ON THE PATH BY WHICH CARBON DIOXIDE PASSES INTO 

 LEAVES DURING ASSIMILATION. 



The experiments on this point are conducted on 

 essentially the same plan as those on respiration, with the 

 difference that the air coming into the capsules is not freed 

 from CCX, but is loaded with a definite percentage of this 

 gas by the "constant C0 2 generator". 



After the leaf has been carefully fitted up in the capsules 

 figured above with the inlet tubes for each side attached to 

 the C0 2 generator and the outlet tubes to the apparatus, the 

 aspirators are allowed to run for a couple of preliminary hours 

 that the strength of CO, may attain constancy. During 

 this time the leaf is kept completely darkened, but must 

 be so arranged that when the covering is removed a bright 

 light falls on the capsules at right angles to the glass side 

 so as to fully illuminate the leaf within. At the end of this 

 time an estimation of the C0 2 passing over either surface 

 is taken for say fifteen minutes, and then the leaf may be 

 illuminated. After a certain length of illumination, a new 

 estimation of the C0 2 is made for the same length of time 

 as before. If any CO, has been taken up by either surface 

 of the leaf, the current from that chamber will show a falling- 

 off in its CO, content ; and, if it does not show this, then 

 no C0 2 has been taken up. Subsequently, the illumination 

 may be altered or stopped, and, after a short time, another 

 estimation made, and so on, ending with a pair of estima- 

 tions in darkness, when, if all has gone well, the original 

 amounts of CO, should be again obtained. 



The details of two experiments are appended ; the first 

 with a leaf of Alisma Plantago which has rather more stomata 

 above than below. 



