Summary 



form 



normal 



Opuntia 



f 





3°4 



BOTANICAL GAZETTE 



[OCTOBER 





of the papilla. The appearance of the abnormal structure shown 

 in fig. 28 seems to favor such a conclusion. 



Among the theories advanced to explain palisade development 

 are the light theory, the transpiration theory, and the lateral 

 pressure theory (2). No attempt was made to separate the light 

 and transpiration factors when the etiolated shoots were trans- 

 planted. Both factors were increased upon transferring the 

 etiolated shoots to the light. Lateral pressure must have been 

 reduced, for the cortical tissues had a minimum of intercellular 

 air spaces in the dark chamber and a maximum in the etiolated- 

 greened condition. Furthermore, the collapse and death of many 

 cortical cells after exposure of the shoots to an outdoor environ- 

 ment must have further reduced lateral pressure. Another 

 factor may have been operative. The partially cutinized epider- 

 mal walls resisted collapse, judging by the appearance of sections 

 of fresh and killed tissues, and this cylinder of epidermis., as it 

 gradually decreased its water loss by increased cutin secretion, 

 must have set up an outwardly directed strain that could not fail 

 to influence the shape of the cells attached to it. 



- 



The appearance of large intercellular spaces in the outer cortex 

 is closely related to palisade development which has already been 

 briefly discussed. Shrinking and subsequent turgidity and the 

 death of cortical cells were important factors in increasing the 

 intercellular spaces and enlarging air chambers. All of these 

 factors are probably active in various plants growing in a natural 

 environment in the southwest. It has already been shown for 

 succulents that shrinkage and expansion are marked with the 

 change from wet to dry seasons (7). An investigation of plants 

 that survive continued dry winds would probably reveal inter- 

 cellular changes in the cortex similar to those observed in these 

 experiments in the etiolated-greened shoots. 





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