INTERNAL ANATOMY OF OKGAIS'S. STEMS. 



529 



in the "bundles, as in the Dicotyledons. In Tradescantia, and in the Grasses 

 also, anastomoses of the isolated fibres take place at the nodes of the stem. 



Dicotyledonous Stems. The stems of Dicotyle- 

 dons, and of Conifers which agree in the main 

 points, are at first of very simple structure, almost 

 resembling those of the Ferns ; but their fibro-vas- 

 cular bundles being of the open or indefinite kind, 

 capable of lateral growth by addition of new ele- 

 ments season after season in their outer regions, the 

 full-grown stems depart widely from the preceding 

 types. 



For purposes of comparison, attention must be 

 confined to shoots or stems of Dicotyledons in their 

 first year of growth, as the formation of annual layers 

 is a phenomenon to which there is nothing corre- 

 spondent in the other Classes (excepting Conifers). 



When a young herbaceous stem of a Dicotyledon 

 is cut across, we find the fibro-vascular bundles 

 standing in a circle round a central parenchymatous 

 mass, the pith, and enveloped by a cellular rind (fig. 

 577). The bundles run in tolerablystraightvertical 

 courses and anastomose freely ; a certain number of 

 bundles are distributed to each leaf. 



As the stem increases in age, each fibro-vascular 

 bundle forms a wedge-shaped mass of wood (fig. 

 578) by development of the inner part of the cam- 

 bium-region, and at the same time a layer of liber 

 at the extreme outer side, next the bark. At the 

 close of the first season, therefore, we have a central 

 pith (fig. 578. p}, immediately bounded by the vascular portion of the 

 bundles (called the medullary ^sheath) (m s), from which pass the vessels 

 to supply the leaves ; next come the wedges of wood (w), formed of pro- 

 senchyma (pr) and ducts (d, d) in most Dicotyledons, of prosenchyma 

 alone 'in Conifers, which passes into the cambial or generating layer (c); 

 and this is continuous outside with the ft&er-bundles (/), corresponding 

 to the wedges of wood : the liber-fibres, like the inner vascular elements, 

 send branches to form part of the ribs of the leaves. 



The fibro-vascular bundles, standing side by side, do not become abso- 

 lutely united, but are separated by thin plates of compressed cellular tissue, 

 running out from the pith to the cortical parenchyma j these plates are 

 called medullary rays (fig. 579). 



The liber-portions of the bundles are associated with rows of clathrate 

 cells (p. 487), and frequently with latex-canals (p. 513), and they are sur- 

 rounded by a layer of parenchyma, composed of cells filled with sap and 

 containing chlorophyll, the herbaceous or cellular envelope (fig. 578, c p) ; 

 and this is protected externally by the dry suberous layer (s I), whi< h 

 succeeds to the epidermis when the herbaceous shoot acquires a woody 

 character. 



Modifications. Many special modifications of the above type are met 

 with in Dicotyledons. In the Piperacea3 there is a kind of double con- 

 2entric circle of fibro-vascular bundles, the inner circle supplying the 



2M 



Diagram of the arrange- 

 ment of the fibro-vascu- 

 lar bundles in a yearling 

 stem of a Dicotyledon. 



