190 EXCHANGES AND MOVEMENTS OF FLUIDS 



of the unobstructed cross section. 1 This condition is imitated by 

 the arrangement of stomata in leaves, which are not to be con- 

 sidered as simple capillaries in studies upon gaseous interchange. 

 The rate of gaseous interchange between leaves and the air, based 

 upon simple measurements of the stomata will thus be found 

 faulty. 



263. Diffusion of Gases through Coating of Fruits. Smooth 

 both ends of a glass tube 60 cm. long and with a bore 5 mm. in 

 diameter. Fit to one end a bored cork in such manner that the 

 top of the cork and tube shall be flush. Work a surface of soft 

 sealing wax over the cork and edges of the tube, then cut a cir- 

 cular piece of fine wire gauze and cover over the cork and tube, 

 imbedding the gauze in the warm wax. Cut a circular piece of 

 the rind of a squash or pumpkin, trimming away the inner layers 

 until it is not more than 2 mm. thick. Lay on top of the gauze 

 and seal around edges with wax, being careful not to burn the 

 material. Place the tube nearly horizontal with the closed end 

 lowest and fill completely full with distilled water, being careful 

 not to displace the fittings of the closed end by cracking the 

 brittle wax. Now place the finger over the open end and stand 

 upright in a dish of mercury, allowing no air to gain entrance. 

 Displace the mercury in the tube with carbon dioxide, or oxygen, 

 until it is at the same level in the tube and dish. Measure the 

 height of the column of mercury as it rises in the tube, by 

 readings, daily, until it sinks to its former level. Make coincident 

 readings of the barometer. Compare the structure and condition 

 of the plant material at the beginning and end of the experiment, 

 or compare fresh and treated portions. It will be found that 

 carbon dioxide diffuses rapidly through the moist plant material, 

 for four or five days very rapidly, until the column of mercury 

 reaches a height of 9 to 12 cm. and then that the drying out of the 

 material makes cracks or openings through which the atmospheric 



1 Brown, H. T. , and Escombe, F. Static diffusion of gases and liquids in relation 

 to the assimilation of carbon and translocation in plants. Annals of Botany, 14: 

 537. 1900. 



