1914] The Ottawa Naturalist. 67 



iniured, in many cases. We must therefor say, as Hannig has, 

 that wounding as such does not induce abscission, but works in 

 a round-about way. 



The Effect of Temperature upon Abscission. 

 The occuiTence of abscission at the time of the year when 

 the cold is increasing, and the dropping of leaves in great 

 numbers on frosty mornings, must not be thought to indicate 

 that low temperatures cause abscission. Fitting found that 

 Geranium pyrenaicum sheds its petals in a much shorter time 

 (2 . 5 minutes) only when the temperature is raised to over 40*^ C. 

 Lower temperatures, but yet as high as 33° to 34° were necessar}^ 

 for certain other species, and in all cases the rapidity was 

 greatest in a saturated atmosphere. Some species have their 

 reaction time reduced from 2 5 seconds to 60 seconds (Linmn, 

 Verhascmn, etc.). Hannig also found that temperatures higher 

 than the normal laboratory ones caused a more abundant 

 shedding of flowers and that sudden change was more effective 

 than gradual. On the other hand, a lowering of temperatures 

 inhibited it. That the higher, more effective temperatures 

 increased the rate of abscission, and even cause them, as Wiesner 

 (35) has suggested in the case of inner leaves, which may become 

 over-heated does not militate against his explanation of abscission 

 in consequence of frost, which may be procured, according to 

 him,, by the macerating (hydrolysing) effect of organic acids 

 escaping from the frozen cells of the abscission layer or by the 

 dift'erences of tension produced at the limits of the dead leaf- 

 stalk tissues and the still living and turgescent cells of the leaf- 

 base. Entire killing of both leaf and abscission layer may be 

 followed by the rotting away of the tissues, and thus ending in 

 separation. High osmotic pressures, held by Wiesner and his 

 pupils to be important in the case of ordinary abscission, have 

 no place in frost defoliation. 



The Effect of Chemical Agents. 



The air and soil of cities and in the neighbourhood of certain 

 kinds of mills and factories is usually more or less contaminated, 

 and, as a result, there -is much detriment to the health of vegeta- 

 tion. Among the first symptoms to be noticed is the shedding of 

 leaves, and this is doubtless a sensitive, and perhaps a ver}^ 

 sensitive, indication of an abnormal condition. Harvey (36) 

 found, for example, that one part of ethelene in 1,000,000 of air 

 was sufficient to cause abscission of the cotyledons of the castor- 

 oil plant (Ricinus), which is perhaps a more delicate reaction 

 than would be observed in trees. It is significant, however, and 

 we may expect to find similar behaviours in many plants. Even 

 the small amount of ilkiminating gas found in the ordinary air 



