DEVELOPMENT OF STEM. — EOOT. 



415 



Fig. 220. 



tered through nearly the whole of the Cellular matrix (although 

 most abundant towards its exterior) in the former case ; but are 

 limited to a circle within the peri- 

 pheral portion of the Cellular tissue 

 in the latter. It is in the further 

 development which takes place during 

 succeeding years in the "Woody Stems 

 of perennial Exogens, that those cha- 

 racters are displayed, which separate 

 them most completely from the Ferns 

 and their allies, whose stems contain 

 a cylindrical layer of Fibro-Yascular 

 bundles, as well as from (so-called) 

 Endogens. For whilst the Fibro- 

 Yascular layers of the latter, when 

 once formed, undergo no further in- 

 crease, those of Exogenous Stems are 

 progressively augmented by the meta- 

 morphosis of the Cambium-layer ; so 

 that each of the bundles which once 

 lay as a mere series of parallel cords 

 beneath the Cellular investment of a 

 first-year's stem, may become in time 

 the small end of a wedge-shaped mass 

 of Wood, extending continuously from 

 the centre to the exterior of a Trunk 

 of several feet in diameter, and be- 

 coming progressively thicker as it 

 passes outwards. The fibro-vascular 



bundles of Exogens are therefore spoken of as 'indefinite;' whilst 

 those of Exogens and Acrogens (Ferns, &c.) are said to be 'definite' 

 or 'closed.' 



306. The structure of the Roots of Endogens and Exogens is 

 essentially the same in plan with that of their respective Stems. 

 Generally speaking, however, the Roots of Exogens have no Pith, 

 although they have Medullary Rays ; and the succession of distinct 

 rings is less apparent in them, than it is in the Stems from which 

 they diverge. In the delicate Radical Filaments which proceed 

 from the larger Root-fibres, a central bundle of Vessels will be seen, 

 enveloped in a sheath of Cellular substance ; and this investment 

 also covers-in the end of the fibril, which is usually somewhat 

 dilated, and composed of peculiarly succulent tissue, forming what 

 is termed the Spongiole. The structure of the Radical filaments 

 may be well studied in the common JJucl'iveed, every floating leaf 

 of which has a single fibril hanging down from its lower surface. 



307. The structure of Stems and Roots cannot be thoroughly 

 examined in any other way, than by making Sections in different 

 directions with the Section-instrument. The general instructions 



Portion of transverse section 

 of Burdock (Arctium), show- 

 ing one of the Fibro- Vascular 

 bundles that lies beneath the 

 Cellular integument. 



