$94 Prof. H. Mohl on the Structure of Dotted Vexselx. 



tion on which succeeding phytotomists might have built, and 

 which required only a slight modification to be consistent 

 with facts. This however did not take place, but certain 

 German phytotomists put forth a string of notions which 

 were so many retrograde steps in the knowledge of these 

 vessels. 



Sprengel, ' Anleit. zur Kenntniss der Gewachse,' 1802, vol. 

 i. p. 103, first laid the foundation for many later erroneous 

 notions, by deriving the dotted vessels, which he moreover 

 confounded with the scalary vessels, from a confluence of 

 the spiral threads. He was acquainted with the articulations, 

 and attributed them to a strong contraction at different in- 

 tervals. 



Another view which has lately met with many advocates 

 originated with Bernhardi, 'Ueber Pflanzengefasse/ p. 35. 

 He had the merit of discovering the outer membrane of spi- 

 ral vessels, and referred equally the formation of scalary and 

 dotted vessels to spiral vessels, but in a different way from 

 Sprengel. He supposed the dots to be the isolated fragments 

 of a broken spiral thread. 



Treviranus, c Vom inwend. Bau der Gewachse/ p. 55, de- 

 served especial praise, as regards the intimate knowledge of 

 the structure of vessels, for his discovery of the composition 

 of these organs of distinct coats. He considered the vascu- 

 lar tubes {Gefdssschlduche) as metamorphosed ligneous tissue 

 {Holzzellen), 4 Beitrag zur Pflanzenphysiologie/ p. 1*1, and 

 supposed that there were dissepiments corresponding with the 

 transverse bands, though at a later period they became eva- 

 nescent. As regards the structure of the dots he is uncertain. 

 He is the first who observed that in the wood of sassafras those 

 parts of the vessels which abut on the medullary rays are fur- 

 nished with dots of a different form. He considers the dots 

 in general as elevations, but the latter form as apertures. 



Moldenhauer, 6 Beitrage zur Anatomie der Pflanzen/ p. 

 264, like Sprengel, derived the porous vessels (pordsen Ge- 

 fasse) from spiral and annular vessels, between the threads 

 of which transverse threads had been formed. According to 

 his notion, the threads lay on the outer side of the primary 

 wall of the tube. The assertion, that in the vessels of the 

 lime-tree those sides which abut on another vessel present 

 the form of a porous vessel, while those abutting on cells, on 

 the contrary, have the structure of a spiral vessel, has been 

 unjustly called in question by some later phytotomists. 



G. R. Treviranus, ' Vermischte Schriften/ vol. i. p. 149, ex- 

 plained the structure of the dots far more correctly than any 

 of his predecessors, when he regarded them as elevations of 



