1914] The Ottawa Naturalist. 71 



plished chiefly by turgor increase, the greater surface curvatures 

 reducing the contact areas and so setting the cells free. Kubart 

 ascribes to chemical alteration a larger share in the process, still 

 attribttting, however, the chief place to turgor, e.g., Syringa. 

 Fitting excludes such alteration in the case of petals studied by 

 him and sees, in a general sudden increase in volume of the cells, 

 the active cause. I find in Ampelopsis and in Impatiens positive 

 evidence against Loewi's view of the matter, separation not 

 being found to involve any change in the shape of the cells, 

 while evidence of chemical alteration, involving both primary 

 and secondary membranes, has been clearly seen. Similarly, 

 abscission of the corolla in Gossypium is without doubt accom- 

 panied bv a decrease in turgor, being otherwise similar in 

 operation to Ampelopsis (leaf, tendril, internode). But in this 

 form the primarv membrane dissolves first, and this is not 

 preceded, at anv rate to an appreciable extent, by alteration of 

 the secondary membrane. Hannig's explanation of the process 

 in Salvia, etc., and Kubart's in part of that in Nicotiana accord 

 with mv own, the latter finding in the organic acid released from 

 the cells involved the agent of dissolution. 



Different are, e.g., Lonicera, Syringa, Hydrangea, and a 

 number of others, chiefly, however, in that the secondary 

 membranes are also attached, bvit more vigorously, and showing 

 marked and measureable swelling. The coUenchyma behaves 

 peculiar^ — the thickened walls resisting attack and lying free 

 in the mucilaginous matrix. Aside from the last mentioned 

 observation" Tison recognizes, in essence, this type of abscission. 

 Kubart would designate it as "maceration." 



Finalh', abscission may be accompanied by growth, usually 

 longitudinal, but, as regards the axis of the organ, may be more 

 or less oblique. The growth (under special conditions very 

 limited in amount) may or may not be accompanied by cell 

 divisions, the occurrence of which has impelled earlier observers 

 to regard the separation tissue as a secondary meristem. Before 

 growth sets in, however, the cell walls are altered chemically 

 (but only slightlv) often in a restricted transverse zone about the 

 cell, and the elongation of the wall takes place here (Loewi). 

 But this chemical alteration may not be so restricted, but may 

 rather be very general, as in the collenchymatic region of the 

 leaf of Populus and of Euonymus, producing a condition directly 

 comparable with that in the leaf of Lonicera, above cited, so that 

 we may agree with Loewi in saying that there is no sharp line 

 of demarkation to be drawn between these processes, the one 



^^Hannig (12, p. 428) appears not to have observed this peculiar 

 behaviour, and no one else, so far as I can determine, has done so. 



