240 Dr. Schleiden on the Anatomico-physiological 



medullary rays. This closing together does not always take 

 place in annual stems, and consequently there exists no differ- 

 ence except the nature of the vessels between the woody frame- 

 work, for example, of Tropceolum majus (unlimited vascular 

 bundles), and the creeping stem of Poly podium ramosum, (li- 

 mited vascular bundles). Only where a defined boundary is 

 produced by a single circle of nearly approximating bundles, 

 can there be any question of bark and pith. There is, ori- 

 ginally, present everywhere only a uniform parenchyma, and 

 it is only afler the development of a part of this into vascular 

 bundles, that the diversity arises, of inclosed substance (pith), 

 and external substance (bark) ; while the medullary rays, 

 which may be traced through all gradations, from narrow la- 

 minae to a continuous communicating parenchymatous mass, 

 traversed merely by some threads of vascular bundles, still 

 preserve the connexion. The dispute about bark, or no bark, 

 in Monocotyledons, is therefore quite foolish, either an empty 

 contention about words, or grounded on the assertion of some- 

 thing decidedly false. Meanwhile, that which has by many 

 people been called the bark of the Monocotyledons, is very 

 different in its origin, structure, and physiological import- 

 ance, from the bark of the Dicotyledons. 



The case of a simple circle of closed vascular bundles only 

 occurs, so far as I know, in the stems of Dicotyledons. In 

 Monocotyledons, on the contrary, it is, I believe, the regular 

 structure of the roots. 



The other case, of several concentric circles of vascular 

 bundles, exists throughout the Monocotyledons, and is to be 

 found among the Dicotyledons in the Piperacece, Nyctaginece, 

 Amaranthacem, Chenopodece, and perhaps in many others, the 

 structure and formation of whose stems are not at present 

 w^ell known. Meanwhile the chief distinction between Mo- 

 nocotyledons, namely that of the closed or unclosed vascular 

 bundles, comes here into action, and gives rise to an en- 

 tirely peculiar woody structure in the before-named Dicotyle- 

 dons. Dr. Robert Brown first drew my attention to this in 

 the stem of a Pisonia, {unknown Burmese tree, in Lindley's 

 ' Introduction to Botany ^ p. 80, fig. 40.). Now, as all these 

 vascular bundles, arranged in various circles, continue to be 



