1920] 



BROWN— STOMATA 



303 





Development of stomata under cortical tissue is a phenomenon 

 that is difficult to explain. Cowles states that although the 

 factors inducing the appearance of stomata are unknown, evidence 

 is not lacking that light favors their development, and that stomata 

 are abundant where transpiration is vigorous, and absent where 

 it is reduced or wanting. Neither factor could have greatly 

 influenced the development of subcortical stomata in a direct way 

 in this case, for light was absolutely excluded and transpiration 

 was much reduced, owing to the increased moisture content of the 

 air in the dark chamber and the covering of cortical cells. In 

 connection with the light factor, however, there is another inter- 

 esting possibility. In MacDougal's experiments, seedlings of 

 Aesculus, whose basal internodes were briefly illuminated, developed 

 laminar bodies in internodes formed some weeks after the stimulus 

 had been given, which were entirely lacking in the absolutely 

 etiolated seedlings. He states that "the stimulative effect of 

 illumination .... may be received by one portion of the body 

 and transmitted to another, and the impulses may even be com- 

 municated to organs not actually formed at the time the stimu- 

 lating rays were received/ 7 The last resort appears to be to 

 ascribe the phenomenon to some internal factor such as is included 

 under the term heredity. 



The papillary structures appear to be peculiar to the etiolated 

 shoots of Opuntia Blakeana. That they result from the division 

 of a cell analogous to a stoma initial seems to be a logical con- 

 elusion, since all stages are found, from the normally developed 

 stoma with two guard cells and two adjacent cells originating 

 from one initial, to the elevated structure consisting of many cells 

 around and below the guard cells, all developed from one initial. 

 The fact that the first division may be a periclinal one appears 

 to be a matter of detail that does not exclude such an interpreta- 

 tion. Some stimulus, possibly previous illumination or some 

 internal stimulus, starts the division process, which is favored by 

 the increased moisture of the dark chamber and by an abundant 

 food supply from the normal shoots at the base of the etiolated 

 ones. Once started, the divisions continue until the cutinization 

 of the epidermal walls offers sufficient resistance to check the growth 



