430 CONSERVATIVE ORGANS. [LECT. VIII. 



which Kieser has named the annular spiral, I can 

 form no idea of the possibility of its transformation 

 from the spiral, the rings being entire and placed 

 at the distance of their diameter from each other; 

 whereas the spires of the simple spiral are in con- 

 tact, and never appear otherwise unless the fibre 

 be drawn out, which is very frequently the case in 

 preparing the section of a stem for the microscope. 

 The hypothesis of Kieser is certainly very im- 

 posing, from the simplicity with which it accounts 

 for the varied form of the sap vessels in the fasci- 

 culi of herbaceous stems ; but my observations 

 prevent me from adopting his conclusions. 



The proper or descending vessels are also of 

 two different kinds, at least in respect of diameter, 

 as is evident in the portion of a transverse section of 

 the stem of Hemlock represented in Plate 9, fig. 

 8 ; but I have not been able to make out satis- 

 factorily the characters of either, except that both 

 are cribriform. Indeed, we can scarcely conceive 

 in what manner these vessels could perform their 

 functions, were their coats completely impervious; 

 for, as they convey the sap, altered in its proper- 

 ties in the leaf, or converted into proper juice, 

 or that fluid from which all the other secre- 

 tions, and even the woody fibre, are produced, 

 there must be some means of communication be- 

 tween them and the cells, in which these secre- 

 tions are deposited: and this would be necessary 



