1919] Coit-Hodgson : Abnormal Shedding of Washington Navel Orange 291 



to 300 per cent. This is followed by dissolution of the gelatinous 

 walls, thus freeing the cells which are now surrounded merely by the 

 very thin and delicate tertiary membrane. No elongation of the 

 tertiary membrane has been observed. Neither has any cell division 

 prior to separation been seen to occur, although immediately following 

 separation this often takes place. So far as ascertained, therefore, 

 abscission in the orange conforms to the usual type, e.g., schizolysis 13 

 representing dissolution of the middle lamellae of the abscission zone 

 cells by hydrolysis with subsequent separation. 



Stimuli Leading to Abscission 



The direct cause of abscission in plants in general is considered to 

 be some stimulus which may be brought into play in a variety of ways, 

 depending somewhat on the nature of the plant involved. Lloyd 14 has 

 taken pains to enumerate some of the different kinds of stimuli which 

 according to various writers have been found to cause abscission. It 

 is our purpose to consider these in turn as a possible cause of abscission 

 in the Navel orange and possibly by elimination to arrive at the true 

 cause or causes involved. 



Mechanical Shock or Traumatic Stimuli 



Fitting 15 has shown that jarring or shaking the flower stalks of 

 Verbascum sp. and Geranium pyrenaicum will result in abscission 

 within a few minutes. "We were unable to produce like results with 

 Citrus by this method. Moreover, abscission has been observed to 

 occur regularly under conditions which would preclude the possibility 

 of this cause being operative with oranges. 



An effort was made to cause abscission by cutting and bruising the 

 young fruits in various ways. The result was a failure in every case. 

 Excission of the style and petals either separately or together, either 

 before or during an thesis, failed to produce abscission. Many of the 

 fruits from which the style had been removed developed to maturity 

 in a normal way. Others abscissed but the reaction time varied so 

 widely as to make it very improbable that the removal of the style 

 was the stimulus involved. 



is Correns, Vermehrung der Laubmoose, Jena, 1899. (Cited from Lloyd.) 

 i* Abscission, Ottawa Naturalist, vol. 28 (1914), pp. 41-52, 61-75. 

 is Untersuchungen iiber die vorzeitige Entbliitterung von Bliiten, Jahrb., f. 

 Wiss. Bot., vol. 49 (1911), p. 187. 



