Spiral Formations in the Cells of Plants, 41 



into broad threaded spiral vessels, and these lastly into nar- 

 row threaded spiral vessels*. Now if the history of the de- 

 velopment of such a fascicle be investigated, it is found that 

 those distant ringed vessels were first formed as spiral vessels ; 

 that then, during the gradual expansion of the internode to 

 which the vascular bundle belongs, the formation gradually 

 progresses towards the exterior, and the last spiral vessel re- 

 mains a narrow threaded one, merely because the longitudi- 

 nal expansion of the cells was already nearly at an end when 

 the spiral deposition took place. The two so-called porous 

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

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

 placed on one another, their walls being perfectly simple ; and 

 only after the expansion in length is terminated, the pores 

 originate on their parietes in the manner described, frequently 

 only in the direction of cells in the interior of the vascular 

 bundle. At the same time the perforation also of the septa 

 takes place, according to the law which seems to me pretty 

 generally valid, that the horizontal septa, or those slightly de- 

 viating from this position, are only perforated with a round 

 aperture, the steeper ascending ones become ladder-like or re- 

 ticulate ; and lastly, the steepest are merely provided with 

 usual pores. 



I conceive it arises from not paying due regard to this 

 history of development that Mohl has not yet recognised the 

 true origin of the annular vessels. I will, therefore, briefly 

 communicate here what I have observed on this point. 



All that Mohl has objected in another place against the erro- 

 neousness of the common view likewise supported by Meyen, 

 that a tearing of the spirals into single coils, and a cohesion 

 of the torn ends to rings takes place, remains perfectly cor- 

 rect ; and I was long convinced of the untenability of that 

 view before I had ascertained the true origin. The difficulties 

 of actual observation of the process lie in what follows : — Of 

 all spiroidea the annular vessels originate exactly from those 

 cells in which a spiral deposition is earliest formed, therefore 

 at a time when they are infinitely small and delicate. This 

 period occurs in the outermost internodes of the bud, and 

 every anatomist is aware of the almost insurmountable diffi- 

 culties which here oppose a more accurate examination. It 

 is true, the delicate indications of the spirals have undoubtedly 

 been recognised everywhere here as of the earliest forma* 



* The same arrangement, with slight modifications, occurs in all vascu- 

 lar bundles of Mono- and Dicotyledons (fig. 12), only that often, in all Di- 

 cotyledons especially, porous formations succeed the narrowest spirals. 



