62 The Ottawa Naturalist. [Aug.- Sept. 



tendency in this direction. I am able to confirm this observation 

 as regards 1912 and 1913. This last year, the plant has retained 

 practically all its leaves except those on the outermost new 

 shoots. It would have occasioned less surprise if the change in 

 tendency of leaf shedding has moved in the opposite direction. I 

 find in this plant also, that a partial abscission layer was formed. 

 The indefinite prolongation of green foliage in plants which 

 show definite periodicity in temperate regions is not difficult to 

 attain under experimental conditions. Flammarion (26) caused 

 seedlings of Quercus robur to retain their leaves by transfer to 

 a greenhouse. By removing the lower leaves from a shoot, 

 Dingier (2 7) was able to postpone abscission of its upper leaves 

 The age of the organ thus enters in as a factor. The cotton plant 

 in the open field begins to show a decreasing activity in mid- 

 summer, even in mid- Alabama. The exact date in 1911 at 

 Auburn was August 14th. When kept under constantly favour- 

 able conditions, as in a greenhouse, its period may be prolonged 

 very greatly. I have grown it for over a year, without any 

 evidence that it could not have been kept in activity for a still 

 longer period. The guayule, Parthenium argentatum, and its 

 congeners, P. hysterophorus, P. lyraHmi and P. incanmn, may be 

 similarly controlled. The shrubby species show a periodicity 

 related to rainfall in their nattiral habitat (the Chihuahuan 

 Desert), but it was found possible in the driest part of a very 

 dry year to stimulate the plant to renewed growth by cutting 

 back the branches, thus showing that the moisture supply alone 

 was the limiting factor, and when the balance between outgo 

 and income was disturbed in favour of the latter, growth became 

 possible. Ampelopsis Veitchii normally sheds tendrils only at 

 the close of the season, but I found them being shed during dry 

 weather from plants which spread over boulders (New York 

 Botanical Garden, Julv, 1913) and were so exposed to high 

 temperature and isolation. Such examples very much strengthen 

 the view that the periodic phenomena of growth and leaf-fall 

 stand in a delicate relation to the environmental factors, a 

 disturbance in any one of which is sufiicient to induce a change 

 in behaviour. The analysis of this relation is possible only, as 

 Klebs has said, by experimental means. We may, therefore, 

 profitably examine this aspect of otxr problem, in order to see 

 what results are at present available. 



The Relation of Abscission to External Factors. 



The intricacy and much detail of the work which has been 

 done, far too little as it,'^at present, may be, will prevent more 

 than a rather curtailed summary, but sufficient, it is hoped, to 

 direct attention to the chief results. 



