302 BOTANICAL GAZETTE [october 



Discussion 



The preceding description of the results obtained in the etiola- 

 tion of Opuntia Blakeana suggest for discussion the factors con- 

 cerned in the development of cuticle; the outline of epidermal 

 cells; the number, origin, and development of stomata; the 

 formation of palisade tissue; and the appearance of air spaces. 



Cuticle formation, it has been suggested, approaches a maxi- 

 mum when transpiration is great in amount, and when it is high 

 in proportion to absorption (2). It has also been stated that 

 cuticle formation is favored by growth in concentrated nutrient 

 media. Both factors were probably operating in these experi- 

 ments. The etiolated shoots certainly transpired much less than 

 they would have done had they been grown in the open, for they 

 promptly wilted when transplanted to the latter environment. 

 Gradual increase in the osmotic pressure of the sap in the joints 

 from which the etiolated shoots obtained their nutrient supply 

 must have occurred, but it was evidently insufficient to induce 

 the development of cuticle. MacDougal found no cuticle for- 

 mation in any of the numerous plants with which he experimented. 



The changes mentioned in connection with the outline of 

 the epidermal cells have been observed by many investigators. 

 Whether light is a factor in determining the shape of the cell 

 walls in the epidermis could not be determined, for an attempt 

 was not made to reduce transpiration when etiolated shoots were 

 removed to an outdoor environment in order to separate the two 

 factors. Mesophytic conditions in the dark chamber may be 



considered favorable to crenated walls in view of the results of 

 other workers (2). 



Decrease in the number of stomata per unit of surface area is 

 in general harmony with the results of numerous investigators. 

 Six or seven times as many epidermal cells per unit of surface 

 appeared on the normal shoots, while the stomata were two to 

 three times as numerous, compared with the etiolated ones. In 

 his experiments with Opuntia Opuntia, MacDougal found that the 

 stomata on the etiolated shoots were reduced in size. Those meas- 

 ured in Opuntia Blakeana were not different in size from the stomata 

 of the normal plant, excepting a few freakish forms of stomata. 





