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114 OEG ANOGRAPHT. 



have no existence. Roots increase in diameter by the forma- 

 tion of annual layers of wood, in the same manner as stems. 



At first the elongating growing extremities of the root consist 

 entirely of parenchymatous cells {figs. Til, a, and 223, c); wood- 

 cells and vessels {fig. 223, fv,) however soon make their ap- 

 pearance, and are constantly added to below by the new tissue 

 formed as the root continues to lengthen. When the root is fully 

 developed, these vessels and wood-cells generally form a central 

 mass or wood, in which there is commonly no pith, and no medul- 

 lary sheath. The medullary rays, however, exist as in the stem ; 

 and externally there is a true bark, which is also covered when 

 young by a modified epidermis without stomata {fig. 224), which, 

 as we have seen, is sometimes called epiblema (p. 48). This 

 epiblema is also furnished with hair-like prolongations, which 

 are commonly termed fibrils ov fibrillcs {fig. 224). These fibrils 

 'are especially evident upon young growing roots, and as these 

 advance in age they perish, while the tissue from which they 

 were prolonged becomes at the same time harder and firmer, and 

 is converted gradually into epiphlceum. 



Eoots have no leaves, and normally no buds, hence they have, 

 s we have seen, no provision for regular ramification ; but they 

 appear to divide and subdivide according to circumstances, with- 

 out any order ; hence while the branches of the stem have a more 

 or less symmetrical arrangement, as already described, those of 

 the root are unsymmetrical. The branches of the root are thus 

 merely repetitions of the original axis from which they are 

 developed, and grow in a similar manner, and like it, have com- 

 monly neither buds nor leaves. To this, however, there are many 

 exceptions, for although the root has no power of forming regu- 

 lar buds, yet adventitious buds may be formed upon its surface, 

 in the same manner as we have seen that under certain circum- 

 stances they may be produced from any parenchymatous tissue. 

 The power which the root thus possesses of forming adventitious 

 buds may be observed in the Plum-tree, the Pyrus japonica, the 

 Moutan Pseony, the Japan Anemone {Anemone japonica), and 

 in many other plants. The latter plant exhibits this tendency 

 in a remarkable degree. 



From the above general description which has been given of 

 the growth, structure, and characteristics of the true root, we find 

 that the chief distinctive characters between it and the stem 

 may be summed up as follows : — 1st, The tendency of the root at 

 its first formation to develop in an opposite direction to the stem, 

 and thus withdraw from the light and air. 2nd, The root does 

 not grow throughout the entire length of its newly formed parts 

 like a stem, but only by additions within its apex. 3rd, the root 

 under ordinary circumstances has no pith or medullary sheath. 

 4th, It has no true epidermis with stomata, but in place of this 

 an integument composed of cells without stomata, to which the 



