FUNDAMENTAL ORGANS. 



123 



cellular and vascular than fibrous) predominates. The coloured and dense bundles, 

 which form a more solid zone towards the periphery, are the lower portions of bundles 

 in which fibres analogous to liber predominate ; and, finally, the less compressed 

 bundles which are usually seen outside of the coloured zone are these same fibres 

 after having branched and spread out, and before being lost in the periphery, which 

 is a cellular zone representing the bark. 



A monocotyledonous stem usually retains about the same diameter throughout. 

 This is because the fibro-vascular bundles, gradually attenuated towards their lower 

 extremity, do not, as in dicotyledons, unite and descend to the bottom of the stem ; 

 hence, any two truncheons of a monocotyledonous stem, being equally rich in bundles, 

 can differ but little in diameter. 



Eoot. In the embryo, the radicle is the simple cellular lower end of the caulicle, 

 which elongates downwards as the latter ascends with its plumule and cotyledons. 

 A monocotyledonous seed usually presents several radicles (fig. 642) ; these are not, 

 however, naked like those of dicotyledons, but are originally enveloped in an outer 

 layer (serving as bark), which they push forward and pierce, emerging from it as 

 from a sheath; whence the name of coleorhiza for this organ (fig. 642). 



Examples have been given of stems emitting accessory or adventitious roots 

 from various parts of their surface ; the structure of these is precisely the same as 

 that of the radicle ;. and they may even be regarded as identical, the radicle being con- 

 sidered as a production of the caulicle, and all roots, whether 

 primary or secondary, as adventitious. 



In its earliest stage the root presents an axis of densely 

 packed cells ; the central of these elongate and form vessels 

 which interlace with those of the stem (fig. 695). The root may 

 be simple or branched, but its branches do not start from the 



axil of a leaf, and are not regularly arranged, like the shoots of c 



the ascending axis. They terminate in fibrils, together called 

 root-fibres, which decay, and are replaced by fresh ones which 

 usually spring from near the base of the youngest branch. Like 

 the stem, the root-branches and fibres are clothed with an 

 epidermis or cuticle, except at the tips, which some botanists 

 call spongioles (SP). The root elongates at the tips of its 

 branches, but not of its root-fibres, which are caducous ; and as 

 the fresh cells of the root-branches are at first deprived of SP_. 

 epidermis, it is supposed that roots absorb moisture from the soil 

 by these, as well as by their root-fibres. 



The fibrous and vascular tissues of roots are the same as 

 those of stems, but no trachese are ever found in them; the 

 cells are distended with juice or filled with fecula (Orchis, 

 fig. 695). 



In dicotyledons, the root is distinguished from the stem by the absence of pith 

 and medullary sheath, and by its axis being occupied by woody fibres ; there is scarcely 

 an exception to this. Its diameter increases, like that of the stem, by the annual 



C 



695. Orchis. 

 Vertical section of a rootlet, 

 much enlarged. The cells 

 (c,c) become gradually organ- 



