190 



PLANT PHYSIOLOGY 



Assimilation is likewise suspended in conifers during the cold 

 season except during short periods of warm weather or when the 

 leaves are warmed in sunlight. Though they preserve their 

 leaves, these organs are inactive during most of the winter, when 

 the stomata are closed. 



Even in the fully opened condition, the stomata occupy but a 

 small part of the leaf surface, not more than 1 to 5 per cent of 

 the whole area of the leaf. It might be thought that the diffusion 



of carbon dioxide into the leaf must be greatly 

 impeded, as the gas has to pass through very 

 small openings. Brown and Escombe showed, 

 however, that an epidermis, perforated by 

 numerous small openings, is less of an obstacle 

 to the diffusion of carbon dioxide than might 

 be supposed. In an experiment conducted 

 with the leaf of Catalpa, it was observed that 

 1 sq. cm. of the leaf's surface absorbed 0.07 cc. 

 of carbon dioxide per hour. An equal surface 

 of an alkaline solution absorbs during the same 

 period 0.12 to 0.15 cc. of CO2, or only twice 

 as much. It must be noted, however, that of 

 1 sq. cm. of leaf surface only 1 sq. mm., or 

 one-hundredth of its area, is occupied by the 

 stomatal openings, the remaining 99 per cent 

 Fig. 55.— Influence i^^ing cuticle. It may be calculated from this 



of coating of stomata ° t • i - . • j. xu 



on the formation of that the carbon dioxide penetrates into the 

 starch in the leaf (/rom gtomata at a rate fifty times higher than the 



rate of absorption by a solution of alkali 

 having an area equal to the area of the stomatal openings. 



This seeming paradox finds its explanation in the peculiarity 

 of the diffusion of gases through small openings. According to 

 Stefan's law, the rate of diffusion through orifices is propor- 

 tional to their area only when their dimensions are very large. 

 With very small openings, the rate of diffusion is a function of 

 their diameters. 



As the number of stomata in a leaf is very large, usually several 

 hundred per square millimeter, the sum of their diameters, in 

 spite of their small size, is considerably greater than the diameter 

 of the leaf. Therefore, diffusion through the membrane per- 

 forated with stomata proceeds almost at the same rate as if this 



