48 The Ottawa Naturalist. [June-July 



The former, according to Reed and Smoot (16) is peculiar in 

 having a layer of compressed pith cells enclosed in the vascular 

 cylinder wliich remains unbroken until touched, although the 

 cortex already shows complete abscission. Impact sufficient to 

 break the vascular tissue allows the expulsion of the fruit, to a 

 considerable distance. Those who have become familiar with 

 the w41d rice recall the fragile character of the stem below the 

 spikelet, which breaks away at a slight touch. It needs not to 

 do more than recall the various behaviours of plants which, 

 like the compositae, set free their one-seeded fruits, sometimes 

 singly, as in Adenostemma (Yapp, 1 7) and indeed in the majority 

 of the family, and sometimes in groups, as in Parthenium, in 

 which each of the five achenes is accompanied by two 

 sterile flowers, while all the remaining staminate flowers are set 

 free en masse (Lloyd, 8).' 



Sinie the abscission of flowers and fruits results from a 

 transverse or oblique ctitting off of the stem, we should expect 

 that the plane of separation would fall at or near the base of 

 an internode. According to Hannig, however, this takes place 

 immediately beneath the flower at the top of the pedicel in 

 several species {Nicotiana, Salvia, etc.), and in the middle of 

 the pedicel in others {Solanum, etc.). In still other species, the 

 abscission plane falls just above a very small bract, these, 

 therefore, according with the general rule. , It may be mentioned 

 in this connection that while separation near the base of the 

 chief axis of the inflorescence may take place (Mirabilis and 

 OxybapJms, Hannig; Impaiiens Sultani), it is no less worthy of 

 note that, in many plants, even after the usefulness of the 

 inflorescence has passed, their chief axes remain as permanent 

 encumbrances. I have been able to find the traces of them in 

 Parthenium argentaium after the passage of five years or longer. 

 Among our own plants one easily finds similar examples, e.g., 

 'Rhus, Xegundo, Syringa, etc. And there are very many plants 

 (palms, agaves, ferns, etc.) to which the leaves cling indefinitely, 

 until they are worn or rotted away. 



A case reqiiiring special explanation is to be found in the 

 Cotton {Gossypium), in which the plane of abscission may pass 

 transverselv through the base of the pedicel, or ma}^ extend 

 downwards along the internode below, even as far as the next 

 node. The diagram (Figure 3c) presents these diversities in 

 graphic form. It has long been a puzzle to those concerned with 

 this plant to account for this peculiarity, recorded in a bulletin 

 on the diseases of the cotton by Atkinson (18) in 1897. To 



" The separation of such ]iarts may be ])assivc and involve no special 

 abscission mechanism of livinc; cells. 



