5l8 SECONDARY CHANGES. 



on the primary xylem-plates, the latter consists of vessels lying somewhat near 

 together, and only separated by narrow one- or few-layered bands of usually non- 

 lignified parenchymatous or fibrous cells. If, as is usually the case here, the main 

 medullary rays are absent, one can scarcely speak of medullary rays at all, they are 

 only indicated by single radial bands of parenchyma. In cases with a relatively 

 small wood (Taraxacum, Rubia, &c. (i) b.) this condition is permanent. Where, oil 

 the other hand, the wood is largely developed (2), as in Rheum, Scorzonera his- 

 panica, Pastinaca, and the swollen roots of Brassica and Raphanus ^, the formation 

 of parenchyma in the ligneous bundle increases with the progressive growth in 

 thickness. The bundle is principally composed of parenchymatous cells with non- 

 lignified walls, decided longitudinal extension, and radial arrangement ; and in this 

 massive thin-wajled parenchyma of the bundle lie the vessels, which form closely 

 united groups, or are ferely quite isolated, and are accompanied by narrow, usually 

 non-lignified fibrous cells. As seen in transverse section they form interrupted radial 

 rows, increasing in number In the centrifugal direction, and concentric zones which 

 are also interrupted, while in 1^ir longitudinal course they form a net with pointed 

 meshes. The medullary rays are inserted between the parenchymatous masses of 

 "wood. Their cells are in many cases distinguished from those of the ligneous 

 bundle by their form, which is usually ralBaily procumbent, and by diiferences in their 

 "contents. This is the case in Rheum, whel^ the procumbent cells of the numerous 

 rays, which are only 1-3 cells in breadth and generally only 6-10 cells in height, 

 are distinguished by containing an abundant yellow fcolouring matter (Chrysophanic 

 acid) from the upright parenchymatous cells of the "baMile, which chiefly contain 

 starch; also in the cultivated root of the Parsnip, &c., '«b«r« the uniseriate to 

 triseriate cells of the medullary rays, densely filled with small starcb-grains, contrast 

 sharply with the narrower, elongated cells of the bundle, in which the "Starch is less 

 abundant. 



On the other hand, many of the cases referred to above, in which there is fco 

 •sharp boundary between the parenchyma of the rays and bundles, belong to this 

 category. As seen in cross-section, the rays may indeed be distinguished in their 

 middle part by the greater radial elongation of their cells, by their general course, 

 &c. ; but they pass over quite gradually into the adjacent parenchyma of the bundle. 

 So, for example, in Scorzonera hispanica, Raphanus, Brassica, and the fleshy swollen 

 roots of Daucus. 



The preceding short statements and examples are only intended to call attention 

 to the most remarkable structural phenomena in the wood of fleshy roots. Reference 

 may be made to the descriptions of officinal roots in the pharmacological literature 

 (Wigand, Fluckiger, and Berg), and especially to the representations i# Berg's Atlas, 

 for illustrations of special characteristics, which are extremely variable in different 

 species, and of the no less varied intermediate forms between those ligneous masses 

 which consist chiefly of parenchyma, and those which aje, in various degrees, more 

 woody, i. e. which agree more in structure with the wood ©f the stem. The variations 

 in the structure of the wood, which may occur within the same species in different 

 individuals, sometimes no doubt as an effect of external conditions, and in the same 



' Compare Nageli, Beitr. I. p. 2j. 



