SECONDARy THICKENING. NORMAL DICOTYLEDONS. 519 



individual in roots of different order and thickness, are considerably greater in these 

 cases than any similar phenomena which have been observed in the wood of the 

 stem. The most conspicuous examples of this are once more afforded by plants 

 which, in their wild form, have thin roots, but in many cultivated varieties are pro- 

 vided with fleshy swollen roots, as species of Brassica, Raphanus, Daucus, &c. In 

 the main root of the wild Daucus Carota, the ratio of the thickness of the wood 

 to that of the surrounding cortex (bast-layer), expressed according to the radii of the 

 transverse section, is 5 : 3. The somewhat firm wood consists in the bundles 'partly 

 of narrow fibrous cells, which are at least 8-10 times as long as broad, and pointed 

 at both ends, and are provided with a moderately thickened membrane with small 

 pits, and partly of rather wide vessels, arranged in radial bands, the walls of which 

 show almost exclusively transverse bordered pits. Between the bundles there are 

 numerous medullary rays, consisting of one or more layers of large, approximately 

 isodiametric, parenchymatous cells. Further details, which might be mentioned, 

 especially as regards the innermost portion of the wood, may here be passed over as 

 non-essential. In the cultivated yellow carrot, the proportion between the radius of 

 the cross-section of the wood and that of the surrounding, chiefly parenchymatous, 

 cortex (bast-layer), is approximately as 1:7. The vessels, at least in the great 

 majority of cases, are reticulated, with transverse meshes, fibrous cells are wholly 

 absent, and are replaced by wide, thin-walled, parenchymatous cells, which abut on 

 one another with horizontal surfaces, and are on the average twice as long as they 

 are broad. Although indications of medullary rays may be recognised, they are not 

 sharply marked off from the parenchyma of the tundles. 



It seems to me remote from the purpose of this book to give a synopsis of all 

 the woods investigatedj iwbich might serve as a key to their identification. Some 

 assistance towards t3ie latter object may be obtained from the preceding pages. 

 Reference may further be made to the literature cited, especially t]iat of Pharmacology; 

 e. g. lo Wiesner's RohstofFe des Pflanzenreichs, Hartig's Forstl. Culturpflanzen, and 

 his treatise, Zur vergl. Anat. der Holzpflanzen, Bot. Ztg. 1859, p. 93, and more 

 especially to Sanio, Ueber die Zusammensetzung des Holzkorper's, &c., Bot. Ztg. 

 1863, p. 401. Joseph MoUer's copious 'BeitrSge zur vergleichenden Anatomic des 

 Holzes,' Vienna, 1876, could not be n>a4^ use of for the present work. 



III. The Bast'. 



Sect. 160. The.cambial ring of Dicotyledons and Gymnosperms with normal 

 growth produces on jts outer side the secondary layers of bast ; these are added to 

 the original ba^l-^one of the stem, which is represented by the phloem-portions of the 

 vascular bundles. A similar process goes on in connection with the primary phloem- 

 groups of the root, in the manner described above. The secondary zones are directly 

 continuous with the original ones, and form, together with the latter, the entire zone 

 or mass 0/ bast. Its external limit is formed by that of the primary phloem-groups and 

 of those portions of the medullary rays which lie between them. By means especially 



[Compare Mbller, Anatomie der Baumrinden, Berlin, 1882.] 



