290 University of California Publications in Agricultural Sciences [Vol.3 



Turning now to a more detailed account of the abscission process 

 itself we find that this subject has received considerable study and 

 investigation. The nature of the abscission process has been studied 

 and described in detail by Hannig 3 and Lloyd 4 for Mirabilis; Balls 5 

 and Lloyd 6 for Gossypium; Loewi 7 for Ampelopsis ; Kubart 8 for 

 Syringa and Nicotiana; Kendall 9 for Nicotiana; Tison 10 and Lee 11 for 

 many other plants, to mention only a few of the researches in this 

 interesting field. While the histology of abscission in Citrus has been 

 described in detail elsewhere 12 by the junior author, it is appropriate 

 that a brief sketch be included here. 



As previously indicated, there are two entirely distinct abscission 

 zones. One is at the base of the pedicel and the other at the base of 

 the ovary. In each case the zone may be considered to be situated at 

 the base of an internode where, on account of the power of forming 

 adventitious buds, it may reasonably be suspected that the tissue 

 retains, to a degree at least, its meristematic nature. The zones con- 

 sist of ten to eighteen layers of cells which in young tissue differ his- 

 tologically very little, if any, from adjacent tissues. In older material 

 differences involving shape, size, and content appear. That in the case 

 of young tissue differences of some kind do exist is shown by the fact 

 that after the stimulus has been applied, yet ten to fifteen hours before 

 visible indications appear, the walls of abscission cells are differentiated 

 by a marked inability to hold certain stains, such as methylen blue. 

 From six to eight hours before abscission the walls of the abscission 

 cells are refractive to a different degree. 



The first indication of actual abscission is a marked swelling and 

 and gelatinization of the walls, which may amount to as much as 200 



3 Untersuchungen iiber das Abstossen von Bliiten, Zeitschr. f . Bot., vol. 5 

 (1913), p. 417. 



* Abscission in Mirabilis Jalapa, Bot. Gaz., vol 61 (1916), pp. 213-30, pi. 13. 



s The Cotton Plant in Egypt (London, Macmillan, 1912), p. 69. 



s The Abscission of Flower-buds and Fruits in Gossypium, and its Eelation to 

 Environmental Changes, Trans. Roy. Soc. Canada, ser. 3, vol. 10 (1916), pp. 55-61. 



7 Blattablosung und verwandte Erscheinungen, Vienna Acad. Proc, vol. 1 

 (1907), pp. 166-983; S-B. d. math.-nat. Kl. d. k. Akad. Wiss., Wien, vol. 116, 

 abt. 1 (1907), pp. 983-1024. 



s Die organische Ablosung der Korollen nebst Bemerkungen iiber die mohlsche 

 Trennungschicht, Ibid., vol. 115 (1906), p. 1491. 



9 Abscission of Flowers and Fruits in the Solonaceae with special reference 

 to Nicotiana, Univ. Calif. Publ. Bot., vol. 5 (1918), pp. 347-428. 



!o Recherches sur la chute des feuilles chez les Dicotyledones, Mem. Soc. Linn. 

 Xormandie, vol. 20 (1900), p. 125. 



" The Morphology of Leaf Fall, Ann. Bot., vol. 25 (1911), pp. 51-106. 



12 An Account of the Mode of Foliar Abscission in Citrus, Univ. Calif. Publ. 

 Bot., vol. 6 (1918), pp. 417-28. 



