STRUCTURE.] DUCTS SCHLEIDEtf'S VIEWS. 79 



into narrow threaded spiral vessels. Now if the history of 

 the development of such a woody bundle be investigated, it 

 is found that the vessels with distant rings w r ere first formed 

 as spiral vessels ; that then, during the gradual expansion of 

 the internode to which the vascular bundle belongs, the for- 

 mation gradually advances towards the exterior, and the last 

 spiral vessel remains a narrow threaded one, merely because 

 the longitudinal expansion of the cells was nearly at an end 

 when the spiral deposit took place. The two so-called porous 

 vessels, on both sides, are, during the whole of this formative 

 process, cylindrical cells, filled with a grumous fluid, and 

 placed over one another, their walls being perfectly simple ; 

 and it is only after the expansion in length is terminated, 

 that the pits originate on their walls. At the same period 

 the perforation of their septa takes place according to a law 

 which seems to me pretty general, that horizontal septa, or 

 such as slightly deviate from this position, are only pierced 

 with a round aperture, those with a more diagonal direction 

 with ladder-like or reticulated apertures; and the most 

 oblique merely with the usual pores ." 



Schleiden conceives that it is from not paying sufficient 

 attention to this progressive development that anatomists 

 have not recognised the true origin of annular ducts, and he 

 thus further explains his own views : Of all spiroids the 

 annular ducts originate most exactly from those cells in 

 which a spiral deposit is earliest formed, and therefore at a 

 time when they are infinitely small and delicate. This occurs 

 in the outermost internodes of the bud, and every anatomist 

 is aware of the almost insurmountable difficulties which are 

 then opposed to a perfectly satisfactory examination. No 

 doubt, the delicate indications of spirals have been recognised 

 everywhere in this part as being among the earliest instances 

 of formation ; but at the moment when the bud begins to 

 develope, the formation usually proceeds so rapidly that the 

 observation of the intermediate stages becomes almost impos- 

 sible. It was therefore most important to find a plant in 

 which such difficulties exist in a slighter degree, and on which, 

 therefore, the process might be accurately observed. For 

 these inquiries the Campelia Zanonia, Rick., and the subter- 



