102 BRIGGSI THE LIVING PLANT AS A PHYSICAL SYSTEM 



Living plants in the dark slowly give off carbon dioxide as a 

 respiration product, similar to animals. Blackmail made use 

 of this phenomenon in a further investigation of the function of 

 the stomata. Two constant currents of air free from carbon 

 dioxide were passed through capsules clamped to the leaf and 

 made tight with wax seals, and the amount of carbon dioxide 

 given up by the leaf determined. A summary of some of Black- 

 man's measurements is presented in table 2. The first three 

 plants are evergreens with a thick cuticle. The leaves of the 

 other plants investigated are thin, and those of Polygonum, in 

 particular, are extremely delicate. In all cases the stomata are 



TABLE 2 



Respiration of C0 2 in Cubic Centimeters per Hour from Upper and Lower 

 Surfaces of Leaves Having Stomata Confined to Lower Surfaces, 



According to Blackman 



PLANT 



Nerium oleander 



Prunus laurocerasus 



Hedera helix 



Platanus occidentalis .... 

 Ampelopsis hederacea. . . . 

 Polygonum sacchalinense 



DEVELOPING LEAVES 



Upper 

 surface 



0.001 

 0.001 

 0.001 



Lower 

 surface 



0.147 

 0.085 

 0.075 



Ratio 



iotr 



_2 

 1 0(7 



10(5" 



MATURE LEAVES 



Upper 

 surface 



0.002 

 0.002 

 0.002 

 0.001 

 0.003 

 0.002 



Lower 

 surface 



0.078 



0.076 



0.054 



0.05 



0.10 



0.03 



Ratio 



3 



io~o" 



3 



4 



2 



TO 75" 



3 



i oxf 



6 

 To~7 



confined to the under side of the leaf. Reference to the table will 

 show that the respiration is confined almost wholly to the 

 stomatal side of the leaf, the maximum respiration from the 

 upper side being only 6 per cent of that from the lower and in 

 most instances far less than this. 



This capsule method was also employed by Blackman for in- 

 vestigating the relative rate of absorption of carbon dioxide by 

 the two sides of the leaf. Air containing a known amount of 

 carbon dioxide was passed through capsules provided with glass 

 faces, the surface of the leaves being brightly illuminated. The 

 air after traversing the capsules was analyzed for carbon di- 

 oxide. In this way, Blackman found that in the case of leaves 



