42 VEGETABLE ORGANOGRAPHY. 



tliougbt that the turns of the spire of the tracheae anas- 

 tomosed together as they advanced in age, whence 

 resulted the appearance of the annular vessel ; then, if 

 the junction continued to increase, the tube would take 

 the appearance of a dotted vessel. 



Rudolphi diifers from Hedwig, for he regarded the 

 tracheae as simple spu-al bands, which form a tube by 

 their circumvolutions ; but he considered that they 

 gradually anastomosed, and thus changed into annular 

 vessels. He affirms, in favour of his opinion, that he has 

 found only spiral vessels in the young growing plants of 

 Alsine mediae Caragana arborescens, &c. 



Mirbel, on the contrary, sets out with the principle, 

 that vessels are a modification of the cellular tissue, and 

 that they are formed of porous cellules : he believes that 

 these cellules, placed end to end, form the strangulated 

 vessels ; and he seems to indicate, without expressly 

 saying so, that these may change into porous vessels ; 

 that, by the near approximation of the pores, they 

 become the vessels which he calls Slit Vessels (vaisseaux 

 fendus), or False Tracheae (J'amses trachees), which only 

 differ from true tracheae in being incapable of unrolling. 

 He admits that all the intermediate states are found in 

 nature ; and that the same tube may, in different parts 

 of its length, offer all these different forms : he calls this 

 the Mixed Tube {tube mixte). But he thinks that each 

 of these states of the vessels is primitive, and not pro- 

 duced by the act of vegetation. 



Treviranus (probably in consequence of the idea 

 originally thrown out by Sprengel) states as the effect 

 of vegetation upon the vessels, that a course of things 

 diametrically opposite ensues. He supposes that the 

 granules which are observed in the cellular tissue, are 

 so many organized vesicles, which, swelling up, form as 

 many new cellules ; that these cellules, according to 



