86 DUCTS SCHLEIDEN'S VIEWS. [BOOK i. 



annular, spiral, and reticulated vessels afford three different 

 forms, very intimately connected, and passing frequently one 

 into the other; but that they must not be considered as 

 temporary degrees of metamorphosis of the same vascular 

 utricle. It is true that a spiral organisation is the ordinary 

 and normal state in the secondary layers of the vessels ; but 

 it is not the only state to be found there. Annular organi- 

 sation occurs as a primary formation, and presents in some 

 degree an intermediate form between the spiral wound to 

 the left and that wound to the right. Moreover, reticulated 

 organisation is also found primitively, sometimes more nearly 

 resembling the pure spiral, and sometimes the annular 

 form." 



To this Schleiden replies that his history of the develop- 

 ment of annular vessels applied only to the simplest case, 

 that of rings arising from a single thread; and he is con- 

 fident that he has not deceived himself in the cases alleged, 

 because his researches were made on vessels which, when 

 mature, are purely annular ; so that he could not but believe 

 that he had before him, not mere persistent modifications of 

 structure, but really stages of transition, even though he 

 could not have regarded the observed forms as actually 

 detected in the act of development ; not to mention, among 

 other circumstances, that the persistent ring is distinguished 

 by the sharpness of its outline, the firmness and clearness of 

 its substance, from the yellowish gelatinous transitory por- 

 tion, with its eroded and defaced margin, observed in the 

 moment of dissolution. 



In conclusion, he points out a source of error against 

 which observers should be guarded, arising from the forma- 

 tion, after the original fibre, of secondary threads, uniting the 

 primary, but certainly of a different nature ; as is proved by 

 their solubility in boiling caustic potash. He also refers, in 

 support of his own views, to the philosophical necessity of 

 limiting the number of principles of interpretation " so long 

 as the impossibility of referring a phenomenon to an old prin- 

 ciple does not imperatively require a new one." 



The researches of Mohl into the true history of Bothrenchym 

 show that on the sides of some kinds of pitted tissue, included 



