297 



cases the tissue which originally filled this lacuna is present, and then on 

 the transverse section it is either exactly like the tissue which surrounds it, 

 or it consists of more thin-walled elements with broader lumina, but 

 between these there are often single thick-walled cells with narrower 

 transverse section and without regular arrangement. Schenk l , who has 

 figured a badly-preserved specimen of this kind, explains the mass which 

 fills the lacuna as bast, and the large elements which adjoin it on the 

 inside as the tracheides of the primary wood, so that the whole of the 

 secondary growth must have originated in extrafascicular cambium. But 

 the original preparations, which he sent me at my request, being all exact 



the ring 



FIG. 39. Structure of Arthropitys. A portion of the transverse section. B fragment of a transverse section of 

 ring of wood with the adjacent parenchyma of the pith, showing two wedges of wood, each of which has a lacuna in 



each wedge. A after Binney (1), slightly magnified. B after Weiss (5). C after Binney (1). 



transverse sections, do not at all justify this explanation ; and I am still 

 less able to declare my assent to it, since Renault 2 has found none but 

 tracheal elements in the portion of the wedge of wood in question in the 

 similar form Astromyelon, and also because the large elements regarded 

 by Schenk as the primary xylem-bundle have proved to be parenchy- 

 matous cells in every case, in which obliquely directed sections have made 

 it possible to determine them exactly. I have had opportunity of ex- 

 amining specimens supplying extremely clear and indubitable proofs of the 

 point, especially in the collection of sections in the Botanical Department of 

 the British Museum. Unfortunately there are not unimportant difficulties 



Zittel (1), p. 237. 



2 Renault (13), t. 7, f. 3. 



