62 2 SECONDARI' CHANGES. 



arranged regularly in radial rows ; it has not been accurately investigated how far 

 this arrangement is disturbed by subsequent dilatation. Sooner or later there 

 appears in the subepidermal layer the formation of the superficial periderm, already 

 mentioned above ; this continues through life, and follows the dilatation of the stem. 

 In Beaucarnea it forms the thick masses of cork which are split up, as the plant grows 

 old, from without inwards, and form a covering for the tuberous base of the stem. 



Sect. 198. Most of the basal tubers of the Dioscoreaceae require more exact 

 investigation in all points. As far as known at present three categories of them may 

 be distinguished: viz. (i) tuberous swollen roots — Dioscorea Batatas; (2) rhizomes 

 with scaly leaves, and composed of many internodes — Dioscorea villosa ; (3) leafless 

 tubers, resulting from the swelling of the first epicotyledonary internode of the 

 seedling — Tamus communis \ also T. polycarpus ', Testudinaria^ and many species 

 of Dioscorea. Only the tubers of category (3) have a cambium, and secondary 

 thickening; and these resemble in the main phenomena those treated in the above 

 paragraphs. The first origin of the cambium is unknown. In the specimens 

 investigated it surrounds the whole lateral surface of the tuber inside a thin paren- 

 chymatous cortex : where the tuber is seated on the ground, as in Testudinaria, with 

 a horizontal base, or, as in Dioscorea sinuata Hort., with an oblique flat base, it is 

 absent towards the base. The form of its cells, the production of secondary wood 

 and sparing cortex, the permanent arrangement in radial rows of the interfascicular 

 elements, and the arrangement and connection of the secondary vascular bundles 

 are fundamentally the same as in the stems described. The interfascicular tissue 

 consists exclusively of thin-walled, starchy, parenchymatous cells often much extended 

 in a radial direction, together with sacs containing raphides. This forms the main 

 mass of the tuber. The thin secondary bundles, which form ^i net in the parenchyma, 

 are collateral. Their xylem consists of elongated tracheides — at least I could not 

 find vascular perforations — their lateral walls being reticulate or scalariform, or being 

 for the most part closely covered with multiseriate, small, transverse, slit-like bordered 

 pits; they are, especially in Tamus and Testudinaria, curved in the most various 

 manner, and rolled one within another. The surface is covered at a very early stage 

 with a periderm, which persists through life, and follows the dilatation, and which 

 forms the usually sclerotic corky crust: it has already been mentioned, p. 113, that 

 in Testudinaria the crust is fissured. 



Sect. 195. A secondary thickening of Moiioco/yledonous roofs is only known in 

 the Dracaenas, in Dr. Draco, marginata, fruticosa, reflexa *, and in Aletris fragrans. 

 "While some of the roots of these plants retain the primary structure unaltered 

 (p. 361), the pericambium of many stronger ones assumes the properties and 

 functions of an extrafascicular cambium. As far as the few and incomplete investi- 

 gations extend, the properties of these and their secondary products are exactly the 

 same as those of the similar parts of the corresponding stem. As far as is known, 

 the secondary thickening always begins first in the root, when it is of a considerable 

 age, and when its primary tissue has long been fully formed, and its endo;'ermal cells 



' Dutrochet, Ols. sur les embrj-ons veg^taux. Nouvelles annales du Museum d'Hist. Nat. IV. 

 1S35). p. 169. ^ Compare Unger, Anat. u. Physiol., p. 2.^9. 



^ Von Mohl, Ueber den Mittelstock von Tamus E'.ephantipes. Verm Schr. p. 186 (^1836). 

 * Caspary, Pringsheim's Jahrb. I. p. 44^. — \Vossidlo, I.e. p. 27. 



