1918] K&ndall: Ahscissimi of Flowers and Fruits in Solunaceac 361 



press the opinion that atmospheric factors work directly in causing 

 "spontaneous'' abscission, although offering, so far as I can see, no 

 evidence for this view. Fitting states that the external influence 

 acts directly in most cases, but that the indirect action is apparent in 

 forms which must build a separation layer before fall can occur. 

 In regard to the action of injury, it seems to be the opinion of most 

 investigators (Hannig, Bacquerel, Gortner and Harris) that the 

 stimulus acts indirectly by interfering in some way with such 

 important physiological processes as transpiration, respiration, or 

 assimilation. On the other hand, if abscission is sometimes a semi- 

 tropistic phenomenon, as Fitting has suggested, it is evident that 

 injury may act directly in causing flower-fall. 



TECHNIQUE 



The results noted below were obtained largely from the examina- 

 tion of microscopic preparations made by the paraffin method, 

 although this method was supplemented by free-hand sections mounted 

 in water. In investigating the condition of the pedicel in some species 

 (Datura sp.. Petunia sp. and several species of Nicotiana) only free- 

 hand sections were examined. For most microchemical studies fairly 

 thick, free-hand sections are preferable. The material for sectioning 

 in paraffin was killed and fixed in various concentrations of the 

 ehromo-acetic series and dehydration and infiltration were, in general, 

 carried on very slowly. The free-hand sections were mounted in water 

 without killing. 



In cutting longitudinal sections of any kind all the pedicels were 

 oriented so that the sections were cut parallel to the main stem of the 

 inflorescence, in the plane formed by the pedicel and stem taken 

 together. In studying the histology of the pedicel and the cytology 

 of the separation layer and in studjdng the method of cell separation, 

 these longitudinal sections were supplemented by cross sections in 

 series through the base of the pedicel. It was impossible to cut very 

 thin, longitudinal sections in paraffin without crushing or breaking 

 the cells ; most of these sections therefore were cut from lOfx. to 15/u, in 

 thickness. For a similar reason, it was found necessary to cut thick 

 sections (20)u,to 25ju,) of the pedicels of fruits in which mechanical tissue 

 had developed. It was possible, however, to cut excellent paraffin 

 sections from 5^ to 7/x in thickness in cross-section or longitudinally 

 through the small cells of the separation zone. Since the cells of the 



