710 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



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 disposition of the fibro- 

 vascular layers which are scattered through nearly the whole of 

 the fundamental tissue (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 fibro- 

 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 epiderm 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 

 upwards. The fibro- vascular bundles of exogens are therefore 

 spoken of as ' indefinite ' or open, whilst those of endogens and 

 vascular cryptogams (ferns, &c.) are said to be ' definite ' or closed. 

 The open nbro-vascular bundles of exogens and of gymnosperms 

 may be stated to consist of three distinct parts : the xylem portion, 

 which consists chiefly of ducts, of the nature of spiral, annular, or 

 pitted vessels, and which is the portion of the bundle nearest to the 

 centre of the organ ; the phloem or ' bast ' portion, which consists 

 largely of prosenchymatous cells, among which are almost always 

 sieve-tubes with their sieve plates, and which is the peripheral 

 portion of the bundle ; while between them is the formative cam- 

 bium, from which fresh xylem is constantly being formed on one 

 side, fresh phloem on the other side. The closed bundles of 

 endogens and of vascular cryptogams consist of xylem and phloem 

 only. When the xylem and phloem portions of fibro-vascular 

 bundle lie side by side, as is usually the case, the bundle is said to 

 be collateral ; when either portion encloses the other like a cylinder, 

 it is concentric. 



The structure of the roots of endogens and exogens is essentially 

 the same in plan as 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 branches 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 branch, which is usually somewhat dilated, and is furnished at 

 its extremity with one or more layers of cells, which are constantly 

 being thrown off, known as the pileorhiza or root-cap. The structure 

 of the branches of the root may be well studied in the common 

 buckweed, every floating leaf of which has a single root hanging down 



