-m.g. 



VHI] ANATOMY OF BLADDERWORTS 107 



of the available evidence regarding the nature of the organs in 

 the Bladderworts, seems to be that in the present state of our 

 ignorance the attempt to fit so elusive 

 a genus into the Procrustean bed of 

 rigid morphology, is doomed to failure. 

 It is probably best, as a purely provisional 

 hypothesis, to accept the view that the 

 vegetative body of the Utricularias par- 

 takes of both stem nature and leaf nature. 

 How such a condition can have arisen, 

 historically, from an ancestor possessing 

 well-defined stem and leaf organs, remains 

 one of the unsolved mysteries of phylo- 

 geny. 



The anatomy 1 of the water Utricu- 

 larias, though showing some curious 

 features, is less anomalous than their 

 morphology. In the stem of U. vulgaris, 

 the tracheids, of which one or more are present, are placed 

 sub-centrally, and surrounded by little groups of phloem. 

 Some degree of dorsiventrality is given to the structure by 

 the thin-walled character of the small lower sector of the 

 vascular cylinder in which the tracheids lie, while the con- 

 junctive tissue of the rest of the stele, towards the upper 

 side of the axis, is fibrous. The tracheal elements are of 

 the nature of "imperfect vessels," being formed from a file of 

 superposed cells, with imperforate, oblique, separation walls. 

 The incompleteness of the conducting elements is probably to 

 be associated with the relative unimportance of the transpira- 

 tion stream in a rootless submerged plant. The vascular cylinder 

 is surrounded by an endodermis, and the cortex is lacunar. The 

 structure of the inflorescence-axis differs very markedly from 

 that of the submerged stem ; the tracheids form a discontinuous 

 ring enclosing a large central pith containing phloem islands. 



1 Tieghem, P. van (i 868) and (i 869!), Russow,E. (i 875), Schenck, H. 

 (1886) and Hovelacque, M. (1888). 



b 2 



FIG. 73. Utricularia vul- 

 garis, L. Developing leaf 

 showing two young blad- 

 ders 6j and b. 2 ; m.g., muci- 

 lage gland. (Enlarged.) 

 [Meierhofer, H. (1902).! 



