529 



cases that the leaf scars of the fallen leaves, or those just about to be 

 loosened, were covered, in a number of plants, by a thin layer of ice. Paul- 

 ovvnia, for example, exhibited an especially thick ice crust. Often the 

 leaves were still connected with their scar only by the ice crystals. These 

 ice crystals had l)ecn formed in the abscission layer of the leaves. The 

 columnal structure of the crystals, their cloudiness, produced above the 

 vascular bundles b}- little air bubbles, and their arrangement, ending sharply 

 with the boundary of the leaf scar, favor the view that no considerable 

 masses of cell sap, which had possibly Howed out, ha\e been frozen but that 

 small particles of water pass through the cell walls- exactly at the place 

 where they are observed and are there stiffened to ice. 



Thf formation of ice may often occur very early and thereby cause, 

 when thawing, the fall of lea\es which otherwise would ha\ e remained for 

 some time on the tree and may e\en still be green. Besides this action of 

 the ice lamellae, a premature autumnal defoliation may set in because the 

 leaf is partially or entirely frozen; it, therefore, suddenly becomes function- 

 less and is then pushed oif . 



In autumnal defoliation the loosening of the leaf always takes ])lace in 

 the abscission layer which, according to W'iesner's observations', docs not 

 always arise from a secondary meristem but is often found also as a rem- 

 nant of the primary meristem. In other cases of leaf-fall the process of 

 disarticulation can take place in different tissues. 



If the process of disarticulation within the layer of separation be con- 

 sidered in general, the following modifications will be found, according to 

 \\ iesner-. So strong an osmotic pressure can be produced in the cells of 

 the abscission layer that the tissues separate from one another, leaving 

 smooth surfaces. This we find in defoliation which is the result of excess 

 of water even where this excess arises from abundant watering after a long 

 period of dryness. The phenomena of the dropping of the leaves of 

 Azaleas, Ericas and New Holland plants, so well known to gardeners, after 

 the drying of the root ball, belong here, as does also summer defoliation 

 with the occurrence of rains after a long drought. 



According to AViesner, in autumnal defoliation the macerating action 

 of organic acids comes especially under consideration. He assumes that 

 the surfaces of separation, in death from frost, as a result, have an acid 

 reaction, and explains this by the fact that the frost kills the cytoplasm, 

 thereby making it permeable to the acids which occur in the cell content and 

 then act on the membranes. Oxalic acid may play a great part in this. 

 The above-named investigator laid the stems of various plants in a 2.5 

 per cent, solution of oxalic acid and found that the leaves had loosened 

 within a few days. The stems of plants which form abscission layers at 

 the internodes also disarticulated within a short time. 



1 Wiesner, Julius, tjber Frostlaul>fall nebst Bemerkung-en iiber die Mechanilv 

 der Blattub]osun.a-. Ber. d. D. Bot. Ges. 1905, Part 1, p. 49. 

 - loc. cit. p. 54. 



