• M. Mohl 0)1 the Structure of Annular Vessels. 17 



of rings. On the contrary, I have explained the formation of 

 annular fibres as a mere modification of spiral fibres^ founded 

 upon the fact, that the ascension of a spiral fibre, which can, 

 on the one hand, be extended so far as to take a longitudinal 

 direction, may, on the other hand, be diminished to such a de- 

 gree that its direction may transversely cross the longitudinal 

 axis of the vessel; the consequence of which must necessarily 

 be, rings returning into themselves instead of spiral fibres. 

 On the contrary. Dr. Schleiden thinks that, in the secondary 

 membranes of the utricles of plants, we can, without any 

 exception, demonstrate a spiral disposition of the fibres ; and 

 that annular vessels are formed by spiral vessels which have 

 a constant tendency to unroll themselves, the coils of whose 

 fibre become united here and there by two and two, so as to 

 form perfect rings, which subsequently become isolated by 

 the absorption of the interposed portion of the fibres. This 

 development, he assures us, may be observed in an examina- 

 tion of the annular vessels in their earliest stage. 



The solution of the question as to which of these two the- 

 ories is the true one, will appear to many, in general, more 

 simple and easy than it really is. It might be supposed that 

 by means of a good microscope, used with the necessary skill 

 and patience, the difficulties of research arising from the mi- 

 nuteness and softness of the vessels in their primary develop- 

 ment would be easily surmounted. Such is in fact the case, 

 though, nevertheless, this is not sufficient to place the matter 

 in a clear light ; for the principal difficulty in researches on 

 the development of a vegetable organ arises, in the present 

 as well as in most other cases, from the organ whose develop- 

 ment is to be studied not presenting the same structure under 

 all circumstances, but, on the contrary, presenting in the dif- 

 ferent cases subjected to examination greater or less deviations 

 from the normal type. It is this circumstance which fre- 

 quently hinders us from deciding whether we have a normal 

 development before us, or only an accidental though persist- 

 ent deviation. The observer, not seeing the successive deve- 

 lopment of an organ effected before his eyes, but having to 

 establish his opinion on isolated facts observed at difierent 

 stages of development, is often induced to consider some ac- 

 cidental and unimportant circumstance of greater interest 

 than it really is, and thus founds upon these exceptions, al- 

 though accurately observed, a theory entirely false. Only by 

 researches frequently repeated can such errors be avoided. 



Before passing to the examination of annular vessels, I 

 shall offer some remarks on the fibre of spiral vessels, 



Ann. 6>' Mar/. N. Hist. Vol. viii. C 



