232 MONOCOTYLEDONS DICOTYLEDONS. [BOOK i. 



about bark, or no bark in Monocotyledons, is either an idle 

 quarrel about words, or is grounded on something decidedly 

 false. It is, however, to be borne in mind, that what many 

 have called the bark of Monocotyledons, is very different 

 in its origin, structure, and physiological importance, from 

 the bark of the Dicotyledons. 



A simple closed-up circle of woody bundles only occurs, so 

 far as I know, in the stems of Dicotyledons. In Monocoty- 

 ledons, on the contrary, I believe, it regularly occurs in the 

 roots. 



The case of many circles of woody bundles, occurs through- 

 out Monocotyledons, and is to be found among Dicotyledons, 

 in Pepperworts (Piperacea3), Nyctagos, Amaranths, Cheno- 

 pods, and perhaps many others, the structure of whose stems 

 is at present ill known. Meanwhile, the great distinction of 

 Monocotyledons, namely their limited woody bundles, here 

 comes into play, for the unlimited bundles of the above-named 

 Dicotyledons give rise to a very peculiar woody structure. 

 Dr. Robert Brown first drew my attention to this in the stem 

 of the Pisonia, (figured in Lindley's Introduction to Botany, 

 fig. 42.) There the woody bundles, arranged in various 

 circles, continue to develope until at last they almost form 

 a continuous mass ; hence the parenchym, which previously 

 separated them, is compressed into small insulated patches, 

 which, when the wood is completely formed, are scattered 

 through it in narrow vertical cords (strange), which may be 

 termed, not incorrectly, vertical medullary rays. On the 

 outside of these cords, on the wood, are frequently found 

 spiroids still unaltered and forming the commencements of 

 outer woody bundles. I have traced the whole develop- 

 ment of this singular structure in two species of Pisonia, in 

 Amaranthus viridis, Beta Cicla, Atriplex hortensis, Cheno- 

 podium Quinoa, &c. The structure of many others of the 

 same families, especially Pepperworts, although imperfectly 

 examined by me, proves that this peculiarity is general among 

 them. 



There is a curious kind of wood, which, with perhaps the 

 whole family of Houseleeks (Crassulacese), seems to be con- 

 structed upon the same plan. In the old stem of an Echeveria 



