STRUCTURE.] UNLIMITED BUNDLES. 231 



same manner as in other cells. In fact young cells are 

 constantly formed in either the upper or lower end of the 

 long original cells (pleurenchym,) and in consequence of 

 their extension grow through them, when, coming in contact 

 with the other end of the cell, they seem to call into existence 

 a new cell at the corresponding place in the cell which im- 

 mediately follows. In this difference between limited and 

 unlimited woody bundles consists the only universal distinc- 

 tion between Monocotyledons and Dicotyledons. In annual 

 Dicotyledons the woody bundle, checked in its further 

 development by the death of the plant, has, it is true, some 

 similarity to Monocotyledons ; nevertheless the difference 

 may with care be distinctly perceived, for the formative layer 

 always retains to the last its generating power; and upon 

 this property is, in fact, founded the lignification of certain 

 annual plants, when they are not allowed to flower, as happens 

 in Mignonette and Stocks. 



II. A second distinction among stems, is founded on the 

 number and arrangement of these woody bundles, that is to 

 say upon whether only a simple circle, or several are present. 

 In the first case the woody bundles generally approach each 

 other sooner or later, and thus form a hollow cylinder, pierced 

 from within outwards by bands of compressed parenchyma, 

 called medullary rays. But this closing up does not always 

 occur in annual stems, and consequently, there is no differ- 

 ence,, except in the nature of the vessels, between the woody 

 skeleton of Tropoeolum majus (consisting of unlimited woody 

 bundles), and the creeping stem of Polypodium ramosum 

 (consisting of limited woody bundles). It is only when a 

 well-defined limit is produced by the closing up of a single 

 circle of woody bundles, that there can be a question about 

 bark and pith. In the beginning nothing is present in a 

 stem except uniform parenchym, and it is only after a part 

 of this has grown into woody bundles, that a difference can 

 be found between the central substance (pith) and the external 

 substance (bark); the medullary rays, which may be traced 

 through all gradations, from narrow plates to a continuous 

 parenchymatous mass, traversed by some thread-like woody 

 bundles, still preserving the connexion. Hence the dispute 



