Prof. H. Mohl 0)1 the Structiere of Dotted Vessels. 395 



the walls of the vessels which are hollow on one side, and 

 have in the middle a puncture-like depression {Vertiefung) 

 with a raised border. 



Meyen, ' Phytotomie/ p. 227, followed Bernhardi in the sup- 

 position that the dots are fragments of a broken spiral thread, 

 only he made the matter still worse by considering the thread 

 as the primary and the tube as the secondary formation. 



Link, '^ Annal. d. Sc. Natur.,' t. xxiii. p. 152, likewise derived 

 the dots of the vessels from the fracture of a spiral thread. 

 He considered the spiral thread itself as hollow. According 

 to his views, porous vessels are not peculiar organs, the dots 

 themselves being portions of spirals which are shorter than 

 in the scalaiy vessels ; in other cases they are swollen portions 

 of the hollow spiral thread. 



In certain treatises which appeared in 1831, (' Ueber den 

 Bau der porosen Gefasse. in den Abhandl. der Acad, zu Miin- 

 chen,' vol. i. p. 445 ; 'Ueber den Bau der grossen getupfelten 

 Rohren von Ephedra, in der Linnsea 1831;' 'De Palmarum 

 Structura,' §. 26-29), I endeavoured to prove that the struc- 

 ture of the scalary vessels and dotted tubes is in reality ana- 

 logous to the structure of the dotted cells. I derived the 

 vessels from membranous closed cells, on whose inner surface 

 at a later period membranes and threads are deposited, and 

 whose dissepiments were either completely absorbed or per- 

 forated in a reticulate or scalary fashion. With respect to the 

 dotted vessels, I made it appear that the structure accommo- 

 dated itself to the condition of the neighbouring elementary 

 organs ; that the dots were thinner portions of the wall of 

 the vessel, and the border depended on a cavity situated on 

 the outside of the wall of the vessel. 



Latterly Link, * Element. Phil. Bot.,' ed. sec, vol. i. pp. 117, 

 181, distinguished two forms of vessels under the names of 

 porous and dotted vessels, but the difference assigned is not 

 clear to me. The porous vessels he derived from spiral ves- 

 sels, whose hollow thread at certain points contracted and 

 then became evanescent, so that the individual fragments of 

 the thread lost their connexion with one another. The 

 dotted vessels are beset with dots, which are relics of spiral 

 vessels, which however become invisible. * 



The two latest treatises on the dotted vessels of Meyen, 

 'Neues System der Physiologic,' vol. i. p. 117, and Schleiden, 

 * Flora,' 1839, vol. i. p. 327, nearly agree with each other. 

 Both derive the dots from fissures which the threads of the 

 secondary coat of the tubes leave open at certain places, in 

 which they are not confluent with each other. Both follow 

 my view of the nature of the border. Schleiden makes the 



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