LECT. VIII.] HOLLOW HERBACEOUS STEMS. 429 



the stem. We must, also, bear in mind, that, in 

 his explanation of these diversities, Kieser has a 

 favourite theory to support ; and believes that the 

 simple spiral is the original form of every ve- 

 getable vessel, whether annular or cribriform, and 

 whether existing in trees and shrubs or in herba- 

 ceous plants. But the fallacy of this opinion is 

 easily proved by a good microscope and attentive 

 observation. I am even satisfied that the exa- 

 mination of the vessels in their united state in 

 the fasciculi of herbaceous stems, often leads to 

 erroneous conclusions; for, as the thread form- 

 ing the spiral is more opaque than the membrane 

 forming the membranous cribriform vessels, if 

 one of these be mixed with several spiral ves- 

 sels, it assumes a ribbed character owing to the 

 spirals situated below or behind it being ob- 

 scurely seen through its coat ; thence we con- 

 clude that it is an annular vessel ; and the error 

 is detected only when we can fortunately separate 

 the vessels from each other, and from the sur- 

 rounding cellular matter, an operation which is 

 extremely difficult. Thus, in my dissections of the 

 stem of Bryony, the large vessels in each fasciculus 

 always appeared as punctuated annulars, until 

 accident enabled me to detach one of them from 

 the connecting parts. It appeared, evidently, to 

 be composed of a simple membranous coat, punc- 

 tured in circles round the vessels (see Plate 9, fig. 

 7). With regard to the rigid membranous vessel, 



