386 STRUCTURE OF PHANEROGAMIC PLANTS. 



separated from each other by plates of cellular tissue, which 

 still remain to connect the central and the peripheral portions 

 of the matrix. This first stage in the formation of the Exogen- 

 ous axis, in which its principal parts — ^the pith, wood, hark, and 

 medullary rays — are marked out, is seen even in the stems of 

 herbaceous plants, which are destined to die down at the end of 

 the season (Fig. 179) ; and sections of these, which are very 

 easily prepared, are most interesting Microscopic objects. In 

 such stems, the difference between the " Endogenous" and the 

 " Exogenous" types is manifested in little else than the disposi- 

 tion of the fibro-vascular layers ; which are scattered 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 peripheral 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 characters are displayed, which separate them most com- 

 pletely from the Ferns and their allies, whose stems contain a 

 cylindrical layer of fibro-vascular bundles, as well as from (so- 

 called) Endogens. For whilst the fibro-vascular layers of the 

 latter, when once formed, undergo no further increase, those of 

 Exogenous stems are progressively augmented by the metamor- 

 phosis 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 becoming progressively thicker as it passes out- 

 wards. The fibro-vascular bundles of Exogens are therefore 

 spoken of as "indefinite ;" whilst those of Exogens and Aero- 

 gens (Ferns, &c.) are said to be " definite" or " closed." 



243. 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 dis- 

 tinct rings is less apparent in them, than it is in the stems from 

 which they diverge. In the delicate radical filaments which pro- 

 ceed 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 Duckweed, 

 every floating leaf of which has a single fibril hanging down 

 from its lower surface. 



244. 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 directions 

 already given (§ 107) leave little to be added respecting this 



