MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 375 



even in the stems of herbaceous Plants, which are destined to die down at 

 the end of the season (Fig. 267); 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 type is man- 

 ifested in little else than the disposition of the fibro- vascular layers; 

 which are scattered through nearly the whole of the cellular matrix 

 (although more 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 completely 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 



.FIG: 267. 



Transverse section of the stem of a Portion of transverse 



climbing. plant (Aristolochiaf) from section of Arctium (Bur- 



New Zealand. dock), showing one of the 



Fibro-vaseular bundles 

 that lies beneath the cellu- 

 lar integument. 



iibro-vascular layers of the latter, when once formed, undergo no further 

 increase, those of Exogenous stems are progressively augmented on their 

 outer side by the metamorphosis 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 becom- 

 ing progressively thicker as it passes upwards. The fibro-vascular bundles 

 of Exogens are therefore spoken of as * indefinite;' whilst those of Endo- 

 gens and Acrogens (Ferns, etc.) are said to be ' definite' or 'closed.' 



375. 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 



