51 6 SECONDARV CHANGES. 



be in contact with one neighbouring tracheide, or with two (as is the case where the 

 elements form alternate rows), while in the stem the presence of a single row of pits 

 is the rule. (Comp. p. 494). 



The wood of the root in Dicotyledonous woods is generally also distinguished from 

 that of the stem by its greater 'porosity' and softness. On the one hand, this character 

 is closely related to the changes in the structure of the annual rings which accompany 

 their decrease in thickness, as described above at p. 512. As in the woods investi- 

 gated the middle strong part of the rings is reduced in the slightly developed ones to 

 such an extent that it may even be entirely absent, and these rings thus consist 

 principally of the wide and relatively thin-walled vessels of the spring-boundary, the 

 distinction indicated must necessarily result. In themselves the large vessels of 

 the wood of the root are inferior in average width to those of the stem-wood, 

 in the case of the Ash and Oak. In the Beech, on the other hand, and in a less 

 degree in the Birch and Aspen also, the average width of the internal vessels, 

 even in relatively thick annual rings, is greater than in the stem. This increase in 

 the relative extent of the total area of the cavities of the vessels is, according to 

 V. Mohl, the only anatomical cause of the greater porosity of the wood of the root 

 in the Beech and Aspen. Further, in other cases a more or less considerable 

 increase in the width of tracheides and cells is found (amounting to ^ in Berberis), 

 and a corresponding decrease in the average thickness of their walls. Besides 

 Berberis, this is the case in Fraxinus, Betula, and Quercus. 



Sect. 159. The second of the two extreme cases distinguished within the 

 general plan of structure, namely, an extremely different anatomical composition of the 

 wood of the root, as compared with that of the stem and its branches, is of very 

 general occurrence among herbaceous Dicotyledons, especially perennials and 

 biennials, the roots of which store up reserve substances such as starch, inulin, &c., 

 and apparently large quantities of water. The distinction is no doubt most strikingly 

 marked in the fleshy tap-roots of cultivated plants, Brassica Rapa and Napus, Daucus, 

 Raphanus, &c. ; but these are only special cases of a phenomenon which is of very 

 general occurrence. The most general anatomical character of these roots consists 

 in the reduction of the specific woody elements as compared with the parenchyma. 

 This is brought about in diflferent ways : — 



(i) By feeble development of the entire ligneous body lying inside the cambium 

 as compared with — 



a. The persistent parenchymatous primary outer cortex, or 



b. The relatively very thick, also chiefly parenchymatous, secondary bast. 



(2) By the development of a relatively small quantity of specific woody elements, 

 i. e. vessels and fibres, in the ligneous body, which as a whole is strongly developed. 



As examples of (i) «. the annual subsidiary roots, approaching 2™™ in thickness, of some 

 Asclepiadeae are to be mentioned. In the shrubby Asciepias curassavica the wood of 

 these roots is more than i™'" thick, cylindrical, and similar in structure to the wood of 

 the stem. In the thick roots of the rhizome of A. Cornuti and Vincetoxicum officinale 

 the greatest diameter of the original diarch xylem-plate is less than 0.3™™ ; the breadth 

 of the secondary mass of wood attached to it is only half as much ; all the rest, except the 

 sl'.ght zone of bast, is primary cortical parenchyma. The subsidiary roots of the Piperaceae 

 further belong to this category, as also do those mentioned at p. 355, which show no 

 thickening of their vascular bundles, or mere indications of it. 



