

42 The Ottawa Naturalist. [June-July 



flowers in the hydrocharids in order to effective pollination — 

 these exemplify the same activity leading to renewed life rather 

 than to the mere sloughing of parts moribimd or dead. 



The processes by which these restdts are brought about 

 and the conditions leading to them constitute the subject of 

 this essay. There is also the purpose of presenting a general 

 summary of the problem of abscission as at present imderstood. 

 It will be deemed unnecessary to make an extended historical 

 review of the development of our knowledge, which, from lack 

 of space, must in any event be excluded.^ Only pertinent 

 references, therefore, will be made during the progress of this 

 discussion. Unless specific mention is made, it will be under- 

 stood that the more highly organized plants are meant. 



The Parts of Plants Which May be Shed. 



Aside from the outer layers of the stem, namely, the 

 epidermis and dead cortex, with included tissues, the parts of 

 the plant which may be shed by the process of abscission are 

 transverse segments of the stem, including one or more inter- 

 nodes, either with or without attached flowers; or any lateral 

 organs, either foliar or floral. 



Beginning with leaves as the most familiar examples, we 

 notice further that either the leaf as a whole falls away from 

 the supporting stem, or, when compound, the individual leaflets 

 fall separately (Ash, Horse Chestnut, Boston Ivy, etc.). The in- 

 stances of Ampelopsis Veitchii and Citrus sp. may be especially 

 mentioned since in these forms the apparently sim'ple leaves are 

 separated both at the base of the leafblade and at the base of 

 the petiole. Ampelopsis Veitchii, however, produces trifoliolate 

 leaves on older shoots, and certain Citrus sp. have also com- 

 pound leaves, so that the single blade may be regarded as a 

 terminal leaflet or as a fusion product of three. In the cases 

 which I have studied {Ampelopsis, Vitis, Fraxinus, Aesculus, 

 Negundo) leaves and leaflets present no difference in the method 

 of separation, in concurrence with earlier students (Tison, Loewi, 

 (3) in Citrus), so far as attention has been paid to the matter. 



The position of the plane of separation varies, but is to be 

 found near the base of the organs in question. In the case of 

 the petiole, it may occur at a point further removed from 

 the stem, and thus leave the leaf base clasping more or less com- 

 pletely the axillary bud (Smilax, Philadelphus, Platanus). 

 In Smilax, the leaf is cut off above the tendrils, so that, although 

 these are of foliar origin, they are allowed to remain sujjporting 



Tison (2) 



^ This material may be in large part foimd in the detailed paper of 



a (2). 



