LECT. VIII.] STRUCTURE OF HERBACEOUS STEMS. 417 



the centre of the stem : these are surrounded by a 

 double cluster of entire vessels, which in the trans- 

 verse section appear like small hexagonal cells 

 (Fig. 4, A. e. i.) ; but in the longitudinal (B. e. i.) 

 they display their proper character, although it is 

 not improbable they are divided, at certain dis- 

 tances, by diaphragms which the transparency of 

 their coats conceals. The central vessels are not 

 all of the same structure, the larger being an- 

 nular, with oblong pores (Fig. 5, a.) ; the next size 

 spiral, with the fillet either punctured or studded 

 with glands (Fig. 5, b.), for I have not been able 

 to satisfy myself on this point; and the smallest 

 (Fig. 5, c.) of the same structure as those vessels, 

 which I have already described as existing in the 

 stem of Tradescantia and of some other her- 

 baceous monocotyledons. These consist of iso- 

 lated rings, separated from each other by a space 

 equal to their own diameter, which are apparently 

 intended as a frame- work to keep extended a 

 simple membranous tube in which they are en- 

 closed (Fig. 5, c.). 



Mr. Kieser is the only phytologist who has 

 published any description of the last-mentioned 

 form of vessel; but he regards it merely as the 

 substratum or origin of the annular punctuated 

 spiral*; and, had I seen it in the stem under 



* Mem. stir I* Organisation des Plantes, Haerlem, 181 4-. 

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