347 



falling, falling due to growth, falling due to heat and falling due to frost. 

 Pfeffer^ gives us an insight into the diversity of the causes. "Such a hasten- 

 ing of the leaf-fall is brought about, for example, by insufficient illum- 

 ination, also by an insufficient water provision and by too high a temperature. 

 Not infrequently, however, a premature shedding of the leaves is caused 

 especially by the sudden change of external conditions, which for perti- 

 nent reasons concern first of all the older leaves." As examples of the 

 injurious influence of a sudden change in the amount of transpiration, 

 Pfefi^er cites the sudden loss of leaves in plants as soon as they are brought 

 from the moist greenhouse air into a dry room. Sharp changes of temper- 

 ature, illumination, etc., can act in the same way. 



V. MohF has studied the anatomical processes very thoroughly. 



The shedding of leaves is accomplished by the formation of a trans- 

 verse parenchyma layer at the base of the petiole, as a rule within the leaf 

 cushion, and, in fact, usually where the cork of the bark passes over into 

 the epidermis of the petiole, and in the interior of the petiole tissue, which 

 is produced by a special cell division. The cells of this layer separate from 

 one another in one plane. 



V. Mohl calls the zone in which the layer of separation is formed, the 

 "round-celled layer," because it consists of very short parenchymatous 

 tissue, which toward the leaf body gradually passes over into the elongated 

 cells of the petiole, but is sharply defined on the side toward the bark of 

 the twig. 



In very many cases, a cork layer formed of plate-like 'cork cells, sep- 

 arates the green bark of the branch, rich in chlorophyll and starch, from this 

 short-celled parenchyma of the round-celled layer of the leaf cushion which 

 usually contains no starch, and very little cholorophyll and turns brown 

 at the base at the time of leaf fall. Schacht^ considers this cork sheet, 

 which, at the sides, passes over into the inner cork layers of the bark, to be 

 the cause of the shedding of the leaves. In fact, it may be assumed that if 

 a cork layer be shoved in between the tissue of the bark and that of the 

 petioles, the food supply of the leaf is impoverished and the leaf gradually 

 goes to pieces. Nevertheless, the cork layer is not the cause of the leaf 

 fall, for V. Mohl has shown that it is not formed in many plants which 

 cast their leaves. Thus, for example, no cork layer can be found in ferns 

 with deciduous fronds (Polypodium, Davallia) further, in Gingko biloba, 

 Fagus sihatica, some varieties of oak, Ulmus campestris, Morus alba, Frax- 

 inus excelsior, Syringa vidgaris, Atropa Belladonna, Liriodendron tulipifera, 

 etc. On the other hand, the cork layer is formed in Populus canadensis and 

 r. dilafofa, Alnus glutinosa, fuglans nigra. Daphne Mezereum, Sambucus 

 racemosa. Viburnum Lantana, Lonicera alpigena, Vitis vinifera, Ampe- 

 lopsis quinque folia, Aesculus macrostachya, Pavia rubra and P. lutea, Acer 



1 Pfeffer, Pflanzenphysiolog-ie. II Edition, Vol. 2 (1904), p. 278. 

 - V. Mohl, tJber die anatomischen Veranderungen des Blattgelenkes, welche 

 das Abf alien der Blatter herbeifiihren. Bot. Zeit. 1860, Nos. 1 and 2. 

 •i Schacht, Anatoniie and Physiologie, II, 136. 



