





STUDIES IN VEGETABLE BIOLOGY. 



501 



Studies in Vegetable Biology.— VI. An Investigation into 



the True Nature 

 Ballia callitricha, Ag. 



The Vegetable-Mgirow and 

 CEB Le M. Moobe. F.L.S. 









. 



[Read 6th March, 1890.] 



(Plate XIV.) 



HlSTOlilCAL INTBODUCT 



Disc 



oveked rather more than half a century ago by Th. Hartig *, 

 and studied, among others, by such renowned botanists as 

 Von Mohl, Nageli, Hanstein, De Bary, Janczewski, and Stras- 

 urger, there is yet some doubt, not indeed concerning the mor- 

 phology of sieve-tubes, for that has been the subject of many a fine 

 memoir, but doubt as to the nature of the substance found so 

 constantly upon the sieve-plates, and which, since the appear- 

 ance of Hanstein's f monograph, has been known as callus. And 

 • not only this, but two distinctly opposite views are yet held as to 

 the origiu of this substance, one being that it is formed by simple 

 swelling-up of the sieve meshwork itself, with subsequent coales- 

 cence of the swollen portions into a continuous mass; while, 

 according to other authors, callus owes its origin to the proto- 

 plasm, or, to speak more correctly, the slime of the sieve-tubes, 

 which deposits it upon the sieve-plates. To follow this latter 

 subjeet, first : the older writers, such as Von Mohl % and Nageli §, 

 may be claimed as advocates of the former of these views ; for 

 they seem to consider the sieve itself as the " middle layer " of 

 the iutertubal wall, the two callus-plates forming the wall proper. 

 The next writer on the list, De Bary ||, with his usual caution, 

 refuses to dogmatize on the subject, remarking that it requires 

 further investigation. Three years after the publicatiou of the 

 ' Vergleichende Anatomie » of De Barv, bis pupil Wilhelm f 















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1 







* * Fortschritte der Foratwissenschaft,' 1837. I have not seen this pub- 

 lication. 



t 'Die Milchsaftgefasse und die vorwandten Organe der Kinde,' 1864. 



t Bot. Zeitung, 1855. 



§ Sitzb. der k. barer. Akad. der Wiss. zu Munchen, 1861. 



II Vergl. Anat., Engl, transl. p. 175. ' , 



1 'Beitrage zur Kenntniss des Siebrolirenapparats dicotjler Pflanzen, 

 1880. 



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