Exchange and Movements of Gases and Liquids. 67 



leaves in asparagus. Compare with the structures shown by 

 various types of leaves. The true leaves of the smilax are 

 minute bracts which have ceased to be of functional value 



Fig. 44. Transverse section of cladode of Asparagus medeloides (smilax). After 



Reinke. 



to the plant. Short branches arising from their axils are 

 flattened and expanded as cladodes, and perform some of the 

 functions of the leaf, including those of transpiration and food 

 formation. 



61. Stomatal openings. Note the structure and ap- 

 pearance of the stomata of the species examined. The Open- 



Fig. 45. Transverse section of thallus of Marchantia, showing stoma and the 

 arrangement of chlorophyl-bearing cells beneath. After Kerner. 



ing which the stoma affords is seen to be a slit between two 

 guard cells. (For a complete examination of the structure and 

 action of stomata, see " Practical Plant Physiology," pp. 196- 



