[lloyd] abscission OF FRUITS 19 



Of these records, a tracing of the former is presented in Fig. 1. 

 In Fig. 2 a graph (marked June) is given in which the amounts of 

 growth and shrinkage for successive periods during the eleven days of 

 the record have been plotted. In the inset, the durations of successive 

 daily periods of shrinkage have been expressed in vertical lines. The 

 graph marked "July" integrates the record for the fruit of which the 

 auxograph is exhibited in Fig. 3. 



Mf Hn Mf Kn Mf Nn Mt _ Nn 



i 



1 T ^ 



July 5,1918 Juglan's Calif, quercina tSuf N5 G ■\Gmm dr 

 X lO Temps;, 14--\5°C Nighl" 15-Z3°C Day 



Fig. 3. Growth record of nut No. 6 for nearly four days. 



The time required for the abscission response under stringent 

 conditions is three days. This was determined by placing cut twigs 

 bearing the nuts in a moist chamber. It is known, however, that 

 under the conditions in the field which procure abscission, the response 

 is usually not so prompt.^ This is probably to be explained on the 

 theory that the stimulus is not as a usual thing, a single intense one 

 but less intense and repeated, with cumulative effect. The gradual 

 reduction of soil moisture, or repeated water deficit in the stem, may 

 be conceived as operating in this way. Only when the stimulus con- 

 sists of severe wounding of the appropriate sort, or of some other 

 strong stimulus, does abscission follow with the utmost speed possible. 



With this in mind it is possible to interpret the graphs before us 

 (Fig. 2). We consider the June record first. During the first three 

 days the growth and shrinkage appear to have been quite normal, 

 and accord with MacDougal's observations on the growth of the nuts 

 in which "enlargement begins after noon and continues until sunrise 

 at which time a retardation or shrinkage sets in, which continues until 

 midday."^ In the nuts to which the record in Fig. 1 pertains, however, 

 the periods of shrinkage extended, normally, till the 14 or 15 hour, 

 beginning at about 8 hour. 



The third day was marked by somewhat lessened growth, and the 

 succeeding two by less growth and by daily shrinkage much greater 

 than before. This is obviously to be referred to hindrance to the 



* Lloyd, F. E. Environmental Changes and Their Effect Upon Boll-shedding 

 in cotton (Gossypium herbaceum). Annals N.Y. Acad. Sci. 29: 1-131. 1920. 



* MacDougal., I.e. 



