1914] The Ottawa Naturalist. 43 



the plant. Aside from special cases of this kind, there is some 

 variation of position, as noted by Loewi (3, page 998) in the leaf 

 of Cinnmnommn, in which the abscission plane was found, in a 

 very few instances, to lie in the neighboiirhood of 2 mm. above 

 the expected position. In Hamamelis there are two parallel 

 separation zones a short distance apart. The leaf is first set free 

 by the upper one in the atitumn, and in the early spring follow- 

 ing a short segment is thrown off (Tison 2). Practical identity 

 of position is attained only in those forms p'^ossessing a differen- 

 tiated layer of tissue, such as that in Polygonum, Syrivga and 

 Zizmia, in which it happens that the abscission plane lies, but 

 why it should do so is not cleat. On the other hand we can very 

 surely say that in many, if not in most instances, there is no 

 slightest suggestion in the histological structure of the organ of 

 a specialized abscission layer {e.g., Viiis, Spiraea, Philadelplms, 

 Hydrangea), and we may say, with Loewi, that abscission is a 

 physiological response, adaptive if you please, to stimAdi, and is 

 not conditioned by a predetermined structure. 



The falling away of the leaf, far from being an economical 

 process, is a necessary response to conditions imposed. To be 

 sure, acids of silica and lime usually assumed to be useless, occur 

 in double the usual quantity in falling leaves, while there is a 

 decrease, due to movement into the stem, of nitrogen salts. The 

 conclusions of Ramann, (4) do not, however, accord wholly with 

 those of Combes (1911), who has held that a migration of sub- 

 stancesprior to leaf-fall does not occur, nor are those remaining 

 in the fallen leaf to be considered a priori as non-utilizable. 

 Much, he maintains, remains which might have been used. The 

 leaf may in this respect be comrared to dead and e.Kfoliated 

 bark, which also is not devoid of useful materials. Foods 

 (starch, sugar) are certainly lost in floral parts and in fruits 

 which have suffered abscission — e.g., starch is lost with the 

 corolla of Gossypium. 



The abscission of cotyledons normallv occurs in the aroid 

 Cryplocoryne ciliata andiniheman^Tove (Rhizophora) (Goebel, 6). 

 These slants are viviparous; their embryos withdraw the over- 

 plus of foods from the cotyledons, which, after abscission, 

 remain within the fruit. The seedling then shifts for itself, 

 finding' anchorage in the soft mud of the shore line habitat. 

 There are, however, other viviparous seedlings which are released 

 from the parent plant without separation of the cotvledons 

 {Podocarpiis, Lloyd, 7). 



Such adaptive behaviour may be matched bv examples of 

 disharmony. The "calamander," a sort of teak (Diospyros 

 thrsuta L.), of Ceylon, is one such. Abscission of the cotvledons 



