600 



fibro-vascular bundles are mostly situated toward the exterior, and a 

 fibrous layer is often found beneath the cortical parenchyma. In 

 small rootlets the bundle is sometimes reduced to a single vessel sur- 

 rounded by cellular tissue. A difference of relative position occurs 

 between the elements of the bundles in roots and stems ; the large, 

 porous vessels of the bundles are most external in the stem ; in the 

 root they are the most internal in relation to the axis of the root. 



" Dicotyledonous Roots. — In this class we find the root presenting 

 a great central aggregation of the descending system, corresponding 

 and opposed to the stem. This kind of development of the root gives 

 rise to what is called the caudex or tap-root, and is especially re- 

 markable in. trees, where its ramifications have frequently a definite 

 relation to the size, number and extent of those of the stem. The 

 caudex does not generally become prolonged down very far into the 

 earth, but its branches acquire a great lateral development. Although 

 this corresponds in some degree to the ramification of certain stems, 

 the form and size of the stems and roots do not bear a constant rela- 

 tion. Either of the two systems may be proportionately much more 

 extensively developed. 



" The structure of the root may be considered as resembling ex- 

 actly that of the stem devoid of pith, and consequently without the 

 unrollable spiral vessels of its sheath. This is the real structure in 

 most herbaceous plants, but not in all trees. The walnut, for in- 

 stance, and the horse-chestnut exhibit a pith highly developed, ex- 

 tending to a considerable distance in the root. 



" The growth takes place by the periodical production of new lay- 

 ers of wood and bark over the whole surface. The increase of length 

 is strictly terminal, no interstitial growth of the cells occurring; the 

 ramifications are prolongations from the parenchyma of the cortical 

 system, in which the fibro-vascular bundles are very quickly develop- 

 ed, and become continuous with those of the parent branch. 



" It has been mentioned that adventitious buds may occur under 

 peculiar circumstances upon roots, even as they do in the bark of the 

 stem. Adventitious roots may also be produced from the stem, a 

 striking example of this being afforded by the rooting of cuttings by 

 which plants are propagated in horticulture. The anomalies of stems, 

 such as the coherence of contiguous ramifications into one, the forma- 

 tion of concentric rings of parenchyma, as in certain Menispermaceaj 

 stems and some of the climbing woody Convolvulaceae of Brazil, are 

 frequently repeated in the roots." — p. 65. 



The next section treats of the Leaf, its " anatomical structure," and 



